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

See PR #132823 for a previous change to sys/epoll tests, and prior
changes before that.

ErrnoCheckingTest ensures that errno is properly reset at the beginning
of the test case, and is validated at the end of it. This change removes
many instances of manual errno manipulation.

I normally run my cmake with LIBC_CMAKE_VERBOSE_LOGGING set to ON so I
can debug build issues more easily. One of the effects of this is I see
which tests/entrypoints are skipped on my machine. This patch fixes up
the tests and entrypoints that were skipped, but easily fixed. These
were:

libc.src.pthread.pthread_spin_destroy
libc.src.pthread.pthread_spin_init
libc.src.pthread.pthread_spin_lock
libc.src.pthread.pthread_spin_trylock
libc.src.pthread.pthread_spin_unlock
(entrypoints were just missing)

libc.src.wchar.btowc
(I forgot to finish it)

libc.test.src.sys.statvfs.linux.statvfs_test
libc.test.src.sys.statvfs.linux.fstatvfs_test
(Incorrect includes for rmdir, needed some cleanup)

libc.test.integration.src.unistd.execve_test
(wrong dep for errno)

libc.test.src.math.smoke.fmaf_test
(add_fp_unittest doesn't support flags)

libc.test.src.stdio.scanf_core.converter_test
(needed to be moved away from string_reader, further cleanup needed)

The previous statvfs tests had several issues, this patch updates them
to meet current standards.

When SYS_statfs64 is used, struct statfs64 is used instead of struct statfs. This patch adds a define to select the appropriate struct, similar to how it's done internally.

This patch also enables fstatvfs and statvfs on riscv, which would not be compiled without this change.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.