Deprecating `CHPL_ATOMICS=locks`

[jabraham17]

The Chapel runtime currently maintains 3 different memory layers (CHPL_ATOMICS)cstdlib, intrinsics, and locks. cstdlib is the preferred and default option in most situations, while intrinsics and locks are both portability options for when the backend does not support C11 atomics. In nearly all cases, using intrinsics is the better portability option. The 2 exceptions that I know of are when using the PGI compiler and when using raspberry pi (specifically 32-bit raspberry pi). intrinsics is also usually more performant than locks.

Do we still need to support locks? We do have 2 distinct cases where locks is the only viable option, but recent testing has shown significant issues with locks. What can we do to other existing layers to maintain support for these platforms while still getting rid of locks