Summary:
This patch uses the actual allocator interface to implement
`aligned_alloc`. We do this by simply rounding up the amount allocated.
Because of how index calculation works, any offset within an allocated
pointer will still map to the same chunk, so we can just adjust
internally and it will free all the same.

Summary:
Now that we have `malloc` we can implement `realloc` efficiently. This
uses the known chunk sizes to avoid unnecessary allocations. We just
return nullptr for NVPTX. I'd remove the list for the entrypoint but
then the libc++ code would stop working. When someone writes the NVPTX
support this will be trivial.

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

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>

Summary:
Currently we dispatch the writing mode off of a runtime enum passed in
by the constructor. This causes very unfortunate codegen for the GPU
targets where we get worst-case codegen because of the unused function
pointer for `sprintf`. Instead, this patch moves all of this to a
template so it can be masked out. This results in no dynamic stack and
uses 60 VGPRs instead of 117. It also compiles about 5x as fast.

Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

This reverts commit 1e6e845d49a336e9da7ca6c576ec45c0b419b5f6 because it
changed the 1st parameter of adjust() to be unsigned, but libc itself
calls adjust() with a negative argument in align_backward() in
op_generic.h.

Relates to: #119281

This is a fixup for
https://github.com/llvm/llvm-project/commit/da6d5fa79a558b66c281bed3f5ce848a69a65208.