Add syscall table for x86 and x86_64

I'm wondering if we should use `SYS_write` or `__NR_write`. Both are available to C programs.

There were previous definitions of SYS_gettid and SYS_perf_event_open

Enable preadv and pwritev on the BSDs

Redox: Use c_long instead of usize for off_t

Add missing constants for mac

I didn't find these constants on other systems than mac so only mac is updated...

Add execvpe for support *nix platforms

Add missing if_data and if_msghdr structs

Add more termios s390x constants

These are all that remain for nix to compile on s390x.

These are all that remain for nix to compile on s390x

Add CMSPAR to linux/mips

It needs to be skipped during testing because there's no way to include
a combination of headers that will expose it without giving a "type has
already been defined" error.

merge poll constants in BSD platforms

Sorry for the different PR about the same constants, the miss in the different architectures appeared progressively as I modify nix source to expose the poll function for the different platforms.

Hopefully, it should be the last one !

add poll constants to FreeBSD and DragonFly

add poll constants to Android

Adding `POLLRDNORM`, `POLLRDBAND`, `POLLWRBAND` and `POLLWRNORM` for Android

Add memalign for newlib

Newlib doesn't have `posix_memalign`

add missing socket constants

Add some missing socket constants that are hardcoded by `nix` (nix-rust/nix#647)

I took the opportunity to merge some constants in a upper module when applicable.

Add read/write to Redox

These functions are required for alloc_system now

Various constants for mips64 and s390x

Now that these platforms were added to `libc`, we'd like to support them for `nix`. Here's some missing constants for us. We'll see if this passes CI...

Clean up CI configuration and add s390x

We can't test s390x because qemu segfaults but we can at least verify that it
compiles.

Closes #650

We can't test s390x because qemu segfaults but we can at least verify that it
compiles.

Closes #650

Bump to 0.2.25

To get newlib bindings (#646) and other stuff on crates.io

Remove --privileged from docker

Per #43091

r? @alexcrichton