Allow to prefer more generalized primitives in gcc/generic.h

* src/atomic_ops/sysdeps/gcc/generic.h (AO_test_and_set,
AO_test_and_set_acquire, AO_test_and_set_release, AO_test_and_set_full,
AO_fetch_and_add, AO_fetch_and_add_acquire, AO_fetch_and_add_release,
AO_fetch_and_add_full): Do not define if AO_PREFER_GENERALIZED.
* src/atomic_ops/sysdeps/gcc/generic.h (AO_compare_and_swap): Do not
define if AO_GENERALIZE_ASM_BOOL_CAS.
Implement nop_read/write and and/or/xor_acquire/release/full for AArch64

* src/atomic_ops/sysdeps/gcc/generic.h (AO_nop_read, AO_nop_write,
AO_and, AO_and_acquire, AO_and_release, AO_and_full, AO_or,
AO_or_acquire, AO_or_release, AO_or_full, AO_xor, AO_xor_acquire,
AO_xor_release, AO_xor_full): Implement.
* src/atomic_ops/sysdeps/gcc/generic.h (AO_nop_full): Implement using
__atomic_thread_fence instead of __sync_synchronize.
* src/atomic_ops/sysdeps/gcc/generic.h: Add comment about AO_load_full
and AO_store_full; update TODO items.
Implement double-wide load/store and CAS primitives for AArch64

* src/atomic_ops/sysdeps/gcc/aarch64.h: Include standard_ao_double_t.h
(before inclusion of generic.h).
* src/atomic_ops/sysdeps/gcc/generic.h (AO_double_load,
AO_double_load_acquire, AO_double_store, AO_double_store_release,
AO_double_compare_and_swap): Implement (only if
AO_HAVE_DOUBLE_PTR_STORAGE defined).