Generalize AArch64 add1/sub1 primitives

* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_fetch_and_add1,
AO_fetch_and_add1_acquire, AO_fetch_and_add1_release,
AO_fetch_and_add1_full, AO_fetch_and_sub1, AO_fetch_and_sub1_acquire,
AO_fetch_and_sub1_release, AO_fetch_and_sub1_full): Remove (since
generalized to the same expression).