By the C standard, <locale.h>, <stddef.h> <stdio.h>, <stdlib.h>,
<string.h>, <time.h>, and <wchar.h> require NULL to be defined.

Closes #134118.

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>

Implement the posix function a64l.
Standard:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/a64l.html

This updates the generated stdlib.h and malloc.h headers to
include the subsets of extenion functions declared by glibc that
are also supported by Scudo and that use only simple types.
Scudo's extensions not declared by glibc are omitted.  glibc's
extensions not implemented by Scudo are omitted.  The mallinfo
and mallinfo2 functions are omitted (at least for now) since they
need struct definitions for their return types.

This uses the new merge_yaml_files feature in hdrgen to share the
source of truth for the malloc suite of functions declared in
both stdlib.h and in malloc.h (without either header including
the other).  It also modernizes the malloc.yaml definition a bit,
including dropping the custom template malloc.h.def file in favor
of using the explicit macros list to generate the includes.

Fixes: #124149

This adds a new main command-line entry point for hdrgen, in the
new main.py.  This new interface is used for generating a header.
The old ways of invoking yaml_to_classes.py for other purposes
are left there for now, but `--e` is renamed to `--entry-point`
for consistency with the new CLI.

The YAML schema is expanded with the `header_template` key where
the corresponding `.h.def` file's path is given relative to where
the YAML file is found.  The build integration no longer gives
the `.h.def` path on the command line.  Instead, the script now
emits a depfile that's used by the cmake rules to track that.
The output file is always explicit in the script command line
rather than sometimes being derived from a directory path.

The .yaml files should live next to the corresponding .h.def
files in libc/include/, rather than next to the implementation of
the tool in libc/utils/hdrgen/.  As with the .h.def files, there
is no need for a yaml/ subdirectory under include/.  This simpler
layout is more natural for maintenance and also simplifies build
integration outside the LLVM CMake build.