Upgrade rlimit consts

cc @aidanhs - this should fix the error you are seeing.

Fix type of RLIMIT consts on linux gnu

Add missing TIOCGRS485, TIOCSRS485 constants to Linux

Closes #1094 .

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 #1094 .

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