Remove direct libc_errno.h inclusion and manipulation of libc_errno in
various unit tests, instead relying on ErrnoCheckingTest machinery.

This is the final mechanical change of migrating libc unit tests to
ErrnoCheckingTest - after it, all the unit tests relying on
ASSERT_ERRNO_* macro will be using ErrnoCheckingTest.h

This patch removes some sched.h includes, preferring the proxy headers.
This patch does not clean them all up as some proxy type headers need to
be added, like for `struct sched_param`.

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Having libc_errno outside of the namespace causes versioning issues when
trying to link the tests against LLVM-libc. Most of this patch is just
moving libc_errno inside the namespace in tests. This isn't necessary in
the function implementations since those are already inside the
namespace.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

This patch is large, but is almost entirely just adding casts to calls
to syscall_impl. Much of the work was done programatically, with human
checking when the syntax or types got confusing.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D156950

Along the way, couple of additional things have been done:

1. Move `ErrnoSetterMatcher.h` to `test/UnitTest` as all other matchers live
   there now.
2. `ErrnoSetterMatcher` ignores matching `errno` on GPUs.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151129

Test was incorrectly named/commented after the sched_{set|get}affinity
functions.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148044

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D145222

Add the macro CPU_COUNT as well as a backing function to implement the
functionality.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D135179