Add experimental newlib bindings

I'm not sure how much desire there is for something like this in the libc crate, but I've been working with a newlib-based toolchain for a while and thought I'd throw this PR up and see what happens.

These bindings are more specifically targeted towards the devkitARM toolchain from http://devkitpro.org rather than newlib in general. I'd be happy to try making things more platform and toolchain-agnostic, but I'm not completely sure what the best way to do that is. I can move more of the arch-specific bindings to the `arm` folder, but should there also be a `devkitarm` subdirectory to further separate specific bindings from other newlib implementations?

There's also the question of if the bindings should live in the `unix` directory in the first place. Newlib aims to provide a unix/posix-like environment and it would be nice to inherit common unix definitions by default, but it can target anything from embedded devices to custom userlands, and as such it often lacks APIs that are common in other libc variants, such as pthreads.

Add EXTPROC for android

Needed for nix-rust/nix#527.

DragonFly - Change PTHREAD_STACK_MIN from 1 to 16k

This is probably related to cargo failing on DragonFly when running with
multiple jobs.

The minimum pthread stack size has been increase as shown here [1],
which was needed to fix nodejs.

[1]: http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/592912cde1dc358bf080ae9aff3eca491688f47c

Add missing flags in glibc arm

Adds two missing flags that are defined everywhere except for arm-linux-gnueabi for some reason : SO_PASSCRED and SO_SNDBUFFORCE.

Also adds MAP_32BIT on x86_64-linux-android and i686-linux-android.

This is probably related to cargo failing on DragonFly when running with
multiple jobs.

The minimum pthread stack size has been increase as shown here [1],
which was needed to fix nodejs.

[1]: http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/592912cde1dc358bf080ae9aff3eca491688f47c

Add missing SO_ constants for android

Those constants are used by https://github.com/nix-rust/nix

I pulled them from https://android.googlesource.com/platform/prebuilts/ndk/+/dev/platform/sysroot/usr/include/asm-generic/socket.h

Add missing termios constants for Mac

Didn't add these the first time around, because it says they're unimplemented, but so are the `NL0` and related constants I added. This also makes testing a little easier having these constants available, and they are in the termios headers on Mac.

Update NDK to r15b and add some missing symbols

Use the new unified headers of the NDK and add some missing symbols for Android.

Fixes #632

Fix types of PTHREAD_PROCESS_* constants

Somehow these ended up with wrong types in #633.

Add socket constants for more platforms

Missing a few constants across all nix-supported platforms (see nix-rust/nix#636) so this adds them. This is still a work in progress as I wanted to make sure I didn't break anything doing most of the fixes. I'll come back and finish this up later.

Add constants for termios ioctls

These should be `c_int` from what I could see for Linux and I assumed the others were the same.

Finish off termios (hopefully!)

`cfmakesane` for FreeBSD and `tcgetsid` for everyone!

This doesn't appear on any other platforms

Add pthread_rwlockattr APIs

I'm basically trying to mirror the existing code for pthread_mutexattr.

Hopefully the non-linux parts are correct - finding the right definitions is rather confusing and occasionally even contradicts existing definitions (e.g. [this](https://github.com/practicalswift/osx/blob/a26375e7b3e01c83e26a23977f909438ed9b31b4/src/libpthread/src/internal.h#L295) `pthread_condattr_t` doesn't match #335).

Add constants for sysconf(3) and pathconf(3)

Add more termios constants

Went through and found some more termios constants that were missing and added them. I skipped some platforms (solaris, uclibc) and assumed Android has the same constants are Linux, as I couldn't find a good reference for what Android exposes, but it generally follows linux, so I assumed the same was true for some of those values.

## References:
 FreeBSD:
   * https://github.com/freebsd/freebsd/blob/d39171bb1f2256bd3bf018314aec600f79b89bc6/sys/sys/_termios.h
   * https://github.com/freebsd/freebsd/blob/d39171bb1f2256bd3bf018314aec600f79b89bc6/include/termios.h
 NetBSD: https://github.com/NetBSD/src/blob/0bff031265b50be8e8b7719aed82212928d6c1df/sys/sys/termios.h
 musl: /usr/x86_64-linux-musl/include/bits/termios.h (identical to Linux-x86_64)
 linux: /usr/include/bits/termios.h
 OpenBSD: https://github.com/openbsd/src/blob/5271000b44abe23907b73bbb3aa38ddf4a0bce08/sys/sys/termios.h
 DragonFlyBSD:
   * https://github.com/DragonFlyBSD/DragonFlyBSD/blob/ddad68003a1b070d6955ae737c834cd3267d3ead/sys/sys/_termios.h
   * https://github.com/DragonFlyBSD/DragonFlyBSD/blob/ddad68003a1b070d6955ae737c834cd3267d3ead/sys/sys/termios.h
 Android: Just use the same as Linux
 uclibc: skipped
 haiku-os: http://cgit.haiku-os.org/haiku/tree/headers/posix/termios.h
 mac: /usr/include/sys/termios.h and /usr/include/sys/_types/_posix_vdisable.h
 solaris: skipped
 bitrig (follows openbsd): https://github.com/bitrig/bitrig/blob/d31f5220a98fae4e74abafe27c8d150843cc8105/sys/sys/termios.h

[uclibc] add more structs to x86_64

This adds more structs to the uclibc/x86_64 port.

Add structs defined in linux/input.h

I was using definitions for these structs from `ioctl` but because @cmr decide to deprecate his crate (and yanked all versions :() I think that `libc` is the best place for them.

In original C header, primitive types uses aliases like `__u16` or `__s32`; for now I replaced them with Rust's counterparts but I'm not sure if it is ok, especially because tests were failing for `u64` (`unsigned long long` vs `unsigned long` on x86_64). Also, should I do something special with union in `ff_effect`?

I would like also to add all constants form `linux/input.h` and `linux/input-event-codes.h` if this PR will be accepted.

Add missing Android constant

This pull requests add the `_SC_NPROCESSORS_CONF` constant to `libc` for Android targets. The similar `_SC_NPROCESSORS_ONLN` already existed.

The reason for adding only this constant is to help fix an [issue with `num_cpus`](https://github.com/seanmonstar/num_cpus/issues/34).