Fix test failures for Clang-3.8 and older (Aarch64)

Fix test failures for Clang-3.8 and older (Aarch64)

For older Clang/aarch64 releases, double-wide atomics are no longer
supported.

* src/atomic_ops/sysdeps/gcc/aarch64.h [__clang__
&& !AO_CLANG_PREREQ(3, 9)]: Do not include standard_ao_double_t.h file;
add comment.
* src/atomic_ops/sysdeps/gcc/aarch64.h [__clang__
&& !AO_CLANG_PREREQ(3, 8)] (AO_SKIPATOMIC_double_load,
AO_SKIPATOMIC_double_load_acquire): Do not define and do not undefine
(since generic.h is included only for GCC and for Clang v3.9+).
* src/atomic_ops/sysdeps/gcc/aarch64.h [AO_AARCH64_ASM_LOAD_STORE_CAS]
(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): Do not define if __clang__
&& !AO_CLANG_PREREQ(3, 9).
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_GCC_HAVE_double_SYNC_CAS):
Likewise.