Check if the processor supports compare-and-exchange on 128-bit values.

Before this commit we checked if the compiler supported compare-and-exchange
on 128-bit values. This turned out to be insufficient. This commit strengthens
the check to see if the processor supports the instruction (or built-in). This
check will not work when cross-compiling (will always disable the 128-bit
atomic) so overrides have been added for this case.