Fix assembly comment prefix and identify memory clobbers for AArch64

* src/atomic_ops/sysdeps/gcc/aarch64.h: Revert comments beginning
marker to '//' (since '@' is not supported for A64 assembler).
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_double_load): Add comment
about the need for STXP instruction; remove TODO item.
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_double_store): Add comment
about absence of "cc" in clobber lists; remove TODO item.
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_double_load,
AO_double_load_acquire, AO_double_store, AO_double_store_release,
AO_double_compare_and_swap, AO_double_compare_and_swap_acquire,
AO_double_compare_and_swap_release): Use Q constraint (instead of "r")
for 'addr' argument to instruct the compiler about memory clobbers.