This commit adds 4 Clang builtins for PowerPC AMO load conditional
increment and decrement operations:
__builtin_amo_lwat_cond for 32-bit unsigned operations
__builtin_amo_ldat_cond for 64-bit unsigned operations
__builtin_amo_lwat_cond_s for 32-bit signed operations
__builtin_amo_ldat_cond_s for 64-bit signed operations