[PowerPC] Define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_* macros on all PPC cores

We support all __sync_val_compare_and_swap_* builtins (only 64-bit on 64-bit
targets) on all cores, and should define the corresponding
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_* macros, just as GCC does. As it turns out,
this is really important because they're needed to prevent a bad ODR violation
with libstdc++'s std::shared_ptr (this is well explained in PR12730).

We were doing this only for P8, but this is necessary on all PPC systems.

llvm-svn: 249009