pthread_exit return type fix

Closes: #1520

Sync to rust-lang/libc branch master

Re-enable all roundtrip tests

Fix building error at backtrace

I get this error when builind backtrace:

error[E0308]: mismatched types
  --> /folk/prj-rust-dev/bpang/ala-diab-pb19l/fix_bugs/latest/.cargo/git/checkouts/backtrace-rs-fb1f822361417489/e745627/src/symboli\
ze/dladdr.rs:38:73
   |
38 |                         Some(SymbolName::new(slice::from_raw_parts(ptr, len)))
   |                                                                         ^^^ expected usize, found u64
help: you can convert an `u64` to `usize` and panic if the converted value wouldn't fit

It shows the 'size_t' is expected to be defined as "usize".

defining uintptr_t = usize and intptr_t/ptrdiff_t = isize, and using uintptr_t to define size_t, and intptr_t to define ssize_t.

Sync with rust-lang/libc branch master

updates for vxworks

fix problems found in testing
VxWorks socket and epoll definitions needed by mio and related crates; epoll only supported in VxWorks kernel; just provide stub definitions for user mode for now
Fixed usage of isize
libc VxWorks cleanups and additions
remove copyright notice

2. implement Debug for big size array
3. remove code related to epoll
4. use 'pub enum' for _Vx_semaphore

add isblank

Fixes #1509

Add SCHED_RESET_ON_FORK

This is a fix for #1511, and is valid for musl and glibc.

Test FreeBSD 12 on latest nightly

~~Let's see if [libc update](https://github.com/rust-lang/rust/pull/63806) for Rust fixed it.~~

Fixes https://github.com/rust-lang/libc/issues/1489

freebsdlike: add several utmpx constants

This adds several `utmpx.h` constants for FreeBSD and DragonflyBSD.

Ref: https://github.com/freebsd/freebsd/blob/a1d2b5187332a366a897689c5fb41d38d7e1b0d8/include/utmpx.h
Ref: http://gitweb.dragonflybsd.org/dragonfly.git/blob/HEAD:/include/utmpx.h

Workaround Azure images not supporting rustup self update

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