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

* Get rid of libc_errno assignments in str_to_* __support tests, since
those API have been migrated to return error in a struct instead.
* Migrate tests for atof and to strto* functions from <stdlib.h> and for
strdup from <string.h> to use ErrnoCheckingTest harness.

Summary:
This is the big patch that implements an efficient device-side `malloc`
on the GPU. This is the first pass and many improvements will be made
later.

The scheme revolves around using a global reference counted pointer to
hand out access to a dynamically created and destroyed slab interface.
The slab is simply a large bitfield with one bit for each slab. All
allocations are the same size in a slab, so different sized allocations
are done through different slabs.

Allocation is thus searching for or creating a slab for the desired
slab, reserving space, and then searching for a free bit. Freeing is
clearing the bit and then releasing the space.

This interface allows memory to dynamically grow and shrink. Future
patches will have different modes to allow fast first-time-use as well
as a non-RPC version.

This patch adds the `memalignment` function to LLVM-libc, following its
description in [WG14 N3220,
§7.24.2.1](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=387).

- [x] Add the implementation of `memalignment` in
[`/src/stdlib`](https://github.com/llvm/llvm-project/tree/main/libc/src/stdlib)
- [x] Add tests for `memalignment` in
[`/test/src/stdlib`](https://github.com/llvm/llvm-project/tree/main/libc/test/src/stdlib)
- [x] Add `memalignment` to
[`entrypoints.txt`](https://github.com/llvm/llvm-project/blob/main/libc/config/linux/x86_64/entrypoints.txt)
for at least x86_64 and whatever you're building on
- [x] Add `memalignment` to
[`include/stdlib.yaml`](https://github.com/llvm/llvm-project/blob/main/libc/include/stdlib.yaml)




Closes #132300

---------

Co-authored-by: Joseph Huber <huberjn@outlook.com>