ggml: thread syncronization on Arm

For x86 slaren was genereous enough to add _mm_pause() to the busy
spin wait loop in ggml_barrier(), but everything else just busy
spins, loading an atomic int on every iteration, thus forcing cache
sync between the cores. This results in a massive drop in performance
on my M2-Max laptop when using 8 threads. The closest approximation
to _mm_pause() on Arm seems to be
     __asm__ __volatile__("isb\n");
After adding this to the busy spin loop, performance for 8 threads
recovers back to expected levels.

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