Declare PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY for aarch64

Good day, hackers.

Architecture Reference Manual for ARMv8 B2.2.1 [1] states:

  For explicit memory effects generated from an Exception level the
  following rules apply:
  - A read that is generated by a load instruction that loads a single
  general-purpose register and is aligned to the size of the read in the
  instruction is single-copy atomic.
  - A write that is generated by a store instruction that stores a single
  general-purpose register and is aligned to the size of the write in the
  instruction is single-copy atomic.

So I believe it is safe to define PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY
for aarch64

[1] https://documentation-service.arm.com/static/61fbe8f4fa8173727a1b734e
https://developer.arm.com/documentation/ddi0487/latest

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regards

Yura Sokolov
Postgres Professional
y.sokolov@postgrespro.ru
funny.falcon@gmail.com