powerpc32: use fallback code for 64 bits __sync primitives

Hi,

I was trying to build gnome-builder on my Mac G4 running OpenBSD/macppc with GCC 8, but hit undefined references due to 64 bits __sync_* primitives being used.

While you documented the issue, the fallback code is not used and requires an addition to avoid using __sync_synchronization().

This passes tests. With these changes, i can build and run gnome-builder on that machine.

Your code is built with -std=gnu11- i considered moving the code to C11 __atomic_* primitives that work everywhere, but i may mess up the code, and the change is way more intrusive.

(fyi @ajacoutot)