Clarify the use of external Docker image

Update emsdk to 1.39.20

Bump up to 0.2.76

Let's prevent this from happening again.

libc 0.2.75

Skip test on aarch64 due to https://github.com/rust-lang/libc/issues/1765

PSP: A few corrections to types and function signatures

Add ucontext_t for aarch64-unknown-linux-musl

Move `x86_64-unknown-cloudabi` to tier 3 check

Linux defines a waitid type `P_PIDFD`, for use with process file
descriptors (`pidfd`). Add that constant.

In libc-test, add linux/wait.h to the Linux-specific and
Android-specific headers, to get the definition. Exclude it on Android
and musl for now, though, as the versions in CI don't have it yet.

Add execl* functions on Windows

Expose si_pid and si_uid from siginfo_t as functions

Add CLD_ constants

On Linux, `sys/wait.h` defines a `W_STOPCODE` macro to construct a stop
code from the signal number of a stopping signal. Provide an equivalent
function.

Suggested-by: Ivan Tham <pickfire@riseup.net>

The SIGCHLD variant of the siginfo structure also provides fields for
user and system time; expose those as well.

The first 3 fields of `siginfo_t` have different orders on MIPS. When
casting `siginfo_t` to a different type to access the fields of the
`sifields` union, avoid giving names to the first three fields, since
they're only present for memory layout and shouldn't be accessed from
the casted structure type.

 Add mac/ios pthread_from_mach_thread_np

On Linux, siginfo_t cannot expose these fields directly due to
https://github.com/rust-lang/libc/issues/716 , so expose them as
functions, just like si_addr and si_value.

In order to get alignment correct on both 32-bit and 64-bit
architectures, define an sifields union that includes a pointer field,
to ensure that it has the same alignment as a pointer.

These constants appear in the si_code field of a SIGCHLD signal or
waitid-returned siginfo value.

On Linux, `sys/wait.h` defines a `W_EXITCODE` macro to construct an exit
code from a return value and a signal number. Provide an equivalent
function.

Allows us to get a pthread id from a match thread id.
From pthread.h [1]:

```
__API_AVAILABLE(macos(10.5), ios(2.0))
_Nullable pthread_t pthread_from_mach_thread_np(mach_port_t);
```

[1] - https://opensource.apple.com/source/libpthread/libpthread-416.40.3/pthread/pthread.h.auto.html