[AIX] Update fetch_and_add type

It turns out that the AIX kernel is defining int instead of unsigned int for fetch_and_add.

Legacy XL also defines this to be signed.

https://www.ibm.com/docs/en/aix/7.2?topic=f-fetch-add-kernel-services

So update the type for compat.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D106920