Copy structs from bits/user.h for musl x86_64

While statically compiling a binary with `musl`, I ran into the following error regarding a missing struct:

```
error[E0412]: cannot find type `user_regs_struct` in crate `libc`
  --> src/debug.rs:37:32
   |
37 | fn show_user_regs(regs: &libc::user_regs_struct) -> String {
   |                                ^^^^^^^^^^^^^^^^ not found in `libc`
```

This struct was previously added for `glibc` in #599, but was never added to `musl`, despite the data format being the same in both.

This fix simply copies `user_regs_struct` into the proper location within the `musl` files.

Remove WASI Core API

Closes #1434

This change does not break the backwards compatibility promise since WASI Core API is unstable right now. If applications or libraries want to use Core API directly they should use [`wasi`](https://crates.io/crates/wasi) instead of `libc`.

Blocked by: rust-lang/rust#63676

cc @sunfishcode

Deprecate AIO_LISTIO_MAX

This value can vary at runtime.  Applications should instead use
sysconf(3) with _SC_AIO_LISTIO_MAX.

Deprecate RLIM_NLIMITS

This constant is not stable across OS versions, so it cannot be used in
any backwards- or forwards- compatible way.  It's typically used to size
arrays in the kernel and in debugging utilities that are closely tied to
the OS version.  Since libc is ignorant about OS versions, we shouldn't
even be defining it.

linux/gnu: add utmpname()

This adds `utmpname(3)` on Linux with GNU libc.

Ref: https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/baselib-utmpname-3.html

Adding UTIME_NOW and UTIME_OMIT to dragonfly

Verified in both repos:
 - [musl](https://git.musl-libc.org/cgit/musl/tree/include/sys/stat.h#n70)
 - [dragonfly](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/e7ab884bd49753f8884eb597d10d6569a08fa0df/sys/sys/stat.h#L250)

Update Emscripten system types

These changes bring the types up to parity with recent Emscripten
versions using the upstream LLVM wasm backend. These changes should be
coordinated with the upgrade of rustc's Emscripten support. See
https://internals.rust-lang.org/t/upgrading-rust-s-emscripten-support/10684

Fix the nightly FreeBSD12 toolchain to one without the build.rs bug

This value can vary at runtime.  Applications should instead use
sysconf(3) with _SC_AIO_LISTIO_MAX.

This constant is not stable across OS versions, so it cannot be used in
any backwards- or forwards- compatible way.  It's typically used to size
arrays in the kernel and in debugging utilities that are closely tied to
the OS version.  Since libc is ignorant about OS versions, we shouldn't
even be defining it.

Fix the link_name for fstat and fstatfs on FreeBSD

This adds `utmpname(3)` on Linux with GNU libc.

Ref: https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/baselib-utmpname-3.html

This reverts commit 89e8ae6a.

UTIME_OMIT and UTIME_NOW values are inverted on OpenBSD. correct it

unbreak OpenBSD after #1474

ci: switch to the rust-lang-ci-mirrors bucket for mirrors

Previously mirrors were stored in the rust-lang-ci2 bucket, which is
meant to store temporary data (the CI artifacts). This switches the code
to fetch from the new mirrors bucket.

The old files won't be removed, but they won't be backed up either.

r? @gnzlbg

Previously mirrors were stored in the rust-lang-ci2 bucket, which is
meant to store temporary data (the CI artifacts). This switches the code
to fetch from the new mirrors bucket.

The old files won't be removed, but they won't be backed up either.

Revert breaking musl change

This reverts commit 9df2deb3.