Add spin_delay() implementation for Arm in s_lock.h

Hi,

Have a tiny patch to add an implementation of spin_delay() for Arm64 processors to match behavior with x86's PAUSE
instruction. See negligible benefit on the pgbench tpcb-like workload so at worst it appears to do no harm but should
helpsome workloads that experience some lock contention that need to spin.
 

Thanks,
Geoffrey Blake