[PowerPC] Adjust `MaxAtomicSizeInBitsSupported` on PPC64

AtomicExpandPass uses this variable to determine emitting libcalls or not. The default value is 1024 and if we don't specify it for PPC64 explicitly, AtomicExpandPass won't emit `__atomic_*` libcalls for those target unable to inline atomic ops and finally the backend emits `__sync_*` libcalls. Thanks @efriedma for pointing it out.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D122868