Re: Add spin_delay() implementation for Arm in s_lock.h

Tom, Andres,

I spun up a 64-core Graviton2 instance (where I reported seeing improvement with this patch) and ran the provided regression test with and without my proposed on top of mainline PG.  I ran 4 runs each of 63 workers where we should see the most contention and most impact from the patch.  I am reporting the average and standard deviation, the average with the patch is 10% lower latency, but there is overlap in the standard deviation.  I'll gather additional data at lower worker counts and post later to see what the trend is.

Cmd: postgres=# SELECT test_shm_mq_pipelined(16384, 'xyzzy', 10000000, workers);

Avg +/- standard dev
63 workers w/o patch: 552443ms +/- 22841ms
63 workers w/   patch: 502727 +/- 45253ms

Best results
w/o patch: 521216ms
w/   patch: 436442ms

Thanks,
Geoff