Use a non-locking initial test in TAS_SPIN on AArch64.

Use a non-locking initial test in TAS_SPIN on AArch64.

Our testing showed that this is helpful at sufficiently high
contention levels and doesn't hurt performance on smaller machines.
The new TAS_SPIN macro for AArch64 is identical to the ones added
for PPC and x86_64 (see commits bc2a050d40 and b03d196be0).

Reported-by: Salvatore Dipietro
Reviewed-by: Jingtang Zhang, Andres Freund
Tested-by: Tom Lane
Discussion: https://postgr.es/m/ZxgDEb_VpWyNZKB_%40nathan