Fix GCC 5.x compatibility for AArch64 double-wide primitives

Fix GCC 5.x compatibility for AArch64 double-wide primitives

Small change to make aarch64 support work with gcc 5.x. Otherwise it
demands gcc -latomic and still fails test_stack.

* 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, AO_double_compare_and_swap_full):
Use assembly implementation even for GCC v5+ (not just GCC 4.x).