[Bug c/63368] Provide an implementation for `__sync_val_compare_and_swap_8' on powerpc (32bits)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63368

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
ppc32 doesn't have a 64-bit CAS instruction, so this would need to be done with
a global lock, not an atomic instruction.