Use ErrnoCheckingTest harness added in
d039af33096c0a83b03475a240d5e281e2271c44 for all unistd tests that
verify errno values. Stop explicitly setting it to zero in test code, as
harness does it.

It also verifies that the errno is zero at the end of each test case, so
update the ASSERT_ERRNO_EQ and ASSERT_ERRNO_FAILURE macro to clear out
its value after the verification (similar to how ErrnoSetterMatcher
does).

Update the CMake and Bazel rules for those tests. In the latter case,
remove commented out tests that are currently unsupported in Bazel,
since they get stale quickly.

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 patch provides specific test macros to deal with `errno`.
This will help abstract away the differences between unit test and integration/hermetic tests in #79319.
In one case we use `libc_errno` which is a struct, in the other case we deal directly with `errno`.

Fixes #75261

Building helloworld.c currently errors with "undefined symbol:
__llvm_libc_syscall"

See: https://github.com/llvm/llvm-project/pull/67032

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

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

This patch standardizes the error messages when a syscall is not
available to be in the format: "ABC and DEF syscalls are not available."

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D148373