Haiku: add [get/set]priority()

These methods were added in Haiku R1 beta 2.

Update Windows Server and Visual Studio versions to 2019

Added clock_getcpuclockid to more targets

Previously clock_getcpuclockid was enabled only on linux, with this
change it is enabled on all linux_like and freebsdlike.

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.