Explicitly cast 0 to size_t type to match fread() return type. This
follows the pattern used elsewhere in this file, and fixes
-Wshorten-64-to-32 warnings when building the test.

Reverts llvm/llvm-project#160413

Relates to https://github.com/llvm/llvm-project/issues/119281

Note:

1) As this PR enables `-Werror` for `libc` tests, it's very likely some
downstream CI's may fail / start failing, so it's very likely this PR
may need to be reverted and re-applied.

P.S.

I do not have merge permissions, so I will need one of the reviews to
merge it for me. Thank you!

This reverts commit 92a116c4ef822950f8c57eaa5164c844c73a1f7e with a fix
for fgets test - convert nullptr to fgets return type (char*), since the
matcher is pedantic.

Reverts llvm/llvm-project#143972 - matcher seems to be pedantic for
fgets tests, reverting to verify and fix.

This reverts commit a93e55e57ed00a55f822c64e3520c7c732b58480 and fixes
build and test failures:

* Proper include added to setvbuf_test.cpp
* fgetc/fgetc_unlocked/fgets tests are ported to ErrnoSetterMatcher and
are made more precise. This fixes inconsistencies between expectations
in regular and GPU builds - ErrnoSetterMatcher is configured to omit
errno matching on GPUs, as fgetc implementation on GPU doesn't set
errno, in contrast to Linux.

Reverts llvm/llvm-project#143802. Follow-up fix
3c7af175e51c3ab08ac3c442146c2b822f38c01e wasn't robust enough and itself
got reverted.

Reduce the direct use of libc_errno in stdio unit tests by adopting
ErrnoCheckingTest where appropriate.

Also removes the libc_errno.h inclusions from stdlib.h tests that were
accidentally added in d87eea35fac5a34a841c637db8908128409a184e

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