AArch64 has single-copy 64 bit atomicity

Hi,

Andres mentioned in passing that he'd defined
PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY only on Itanium, PPC and x86 but
not ARM.

I took a look at https://developer.arm.com/documentation/ddi0487/gb/
under "B2.2.1 Requirements for single-copy atomicity" and it seemed
like we should turn this on for __aarch64__.  It goes back to the
original ARMv8-A so should cover all 64 bit ARM systems.