[mips] Use libatomic instead of GCC intrinsics for 64bit

The following GCC intrinsics are not available on MIPS32:

__sync_fetch_and_add_8
__sync_fetch_and_and_8
__sync_fetch_and_or_8
__sync_val_compare_and_swap_8

Replace these with appropriate libatomic implementation.

Patch by Miodrag Dinic.

Differential Revision: https://reviews.llvm.org/D45691

llvm-svn: 355687