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.

https://github.com/llvm/llvm-project/issues/159474

Another try of trying to land
https://github.com/llvm/llvm-project/pull/166382
- Fix some leftover tests checking for specific  errnos
- Guard errno checking tests to not run on the GPU

@michaelrj-google

The patch #166382 fixed most of these, but missed the fprintf_test ones.

Found in testing against abseil. Two bugs were found:

1) SHIFT_AMOUNT in float_converter<long double> would sometimes be
   negative causing an underflow when passed as the amount to left shift
   by for BigInt.
2) is_lowest_block had an off-by-one because it was adding 1 to the
   block index.

Both are fixed and there are new tests to catch regressions.

https://github.com/llvm/llvm-project/issues/159474

Resubmitting https://github.com/llvm/llvm-project/pull/162876 with fixes
as it broke some buildbots:
 - Fix comparisons of integer expressions of different signedness
- Not check for specific errnos in tests, as they might not be available
on all platforms

[#159474](https://github.com/llvm/llvm-project/issues/159474)

- All printf variants set errno and consistently return -1 on error,
instead of returning various predefined error codes
- Return value overflow handling is added

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!