Deprecate fixed width integer type aliases

cc @emilio - I think it makes sense to ensure that the latest released version of bindgen works properly with this change. That is, that even when asked to use C types from, e.g., `libc::`, it does not use aliases for the fixed-width integer C types (e.g. `libc::int64_t`) but uses Rust primitive types instead (e.g. `u64`).

Closes #1304 .

Add statx on Linux

Closes #1178.

Deprecate AF_MAX and PF_MAX

Closes #1304 .

Closes #1178.

Cleanup asmjs

Correct datatype for various termios constants

This was a part of #625, but I moved it to a separate PR as it's a breaking change and those are currently blocked for Tier 1 platforms.

Add F_SEAL_FUTURE_WRITE on Linux/Android

This was added in Linux 5.1 and will only show up in the next glibc
release, thus skip in tests.

This was added in Linux 5.1 and will only show up in the next glibc
release, thus skip in tests.

Re-enable emscripten on CI and cleanup its libc-test

[breaking change] Cleanup linux and update MUSL

* Update MUSL kernel headers to 4.4.2 (non-breaking)
* [breaking] `MADV_SOFT_OFFLINE` is not defined on MIPS
* [breaking] `sendmmsg`/`recvmmsg` take an `unsigned int` flag on MUSL
* [breaking] `pthread_t` is a pointer on MUSL
* `rlimit` resources should use a type alias on GNU (non-breaking)
* Deprecate `SIGNUNUSED` (should use SIGSYS instead)

Manually implement extra traits for `mq_attr` and `sockaddr_nl`

Avoid including padding fields in extra trait implementations as these fields aren't guaranteed to be 0 or some other sensible value.

Closes #1302

Fix SA_* constants for `sigaction` in Android

`nl_pad` field does not contain any actual data, so using it for
comparison or hashing doesn't make sense. Instead manually implement
extra traits ignoring this field.

The `pad` or `__reserved`  fields are not always 0 on some platforms,
so when used in the `PartialEq` implementation being used, fails some
comparisons. This commit manually implements the extra traits to
correct this behavior.

Clean up libc-test/build.rs for FreeBSD and enable testing FreeBSD12 on CI

Deprecate mach APIs: users should use the `mach` crate instead.

See #981, the mach APIs have breaking changes from MacOSX SDK release to release, and that's pretty much what the users are seeing here. We are currently not exposing that many `mach` APIs in `libc`, so this PR deprecates them, forwarding users to use the `mach` crate instead, which provides the mach user-space APIs, is tested against multiple SDK versions, handles removed/deprecated/breaking API changes/etc. Doing all of that in `libc` feels overkill.

Closes #981 .

Add powerpc64-unknown-freebsd target

Per https://github.com/rust-lang-nursery/stdsimd/pull/765 add powerpc64-unknown-freebsd to prevent further breakages.

Fix android SA_* constants

Trying to crosscompile wait-timeout to armv7-linux-androideabi I found that SA_* had different types in libc, but they have to be the same because they are used in bitwise operations.