Summary:
NVPTX does not support external weak symbols being defined, disable this
to get the bots working.

This is a part of allocator patch since I want to make sure the TLS for
allocators are correctly handled.

TLS dtors are not invoked on exit previously. This departures from major
libc implementations.

This PR fixes the issue by aligning the behavior with bionic.


https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/bionic/exit.cpp

I believe the finalization order is also consistent with glibc now:

On main thread exiting:

- pthread_key dtors are not called (unless exiting with `pthread_exit`)
- `__cxa` based tls dtors are called
- `::__cxa_atexit` and `::atexit` dtors are called
- `.fini` dtors are called


![image](https://github.com/user-attachments/assets/737c4845-cab6-47a9-aa00-32997be141bd)

https://github.com/llvm/llvm-project/issues/149993

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