atomic: Check for __sync_val_compare_and_swap instead of __sync_synchronize

Not all gcc configurations have an implementation of all the atomic
operations, and some gcc configurations have some atomic builtins
implemented but not all.

Thus check for the most essential function, whose presence should
indicate that all others are present as well, since it can be used
to implement all the other ones.

Signed-off-by: Martin Storsjö <martin@martin.st>