Support ILP32 in AArch64 assembly routines (GCC)

Support ILP32 in AArch64 assembly routines (GCC)
(back-port of commit 3f41ecb from 'release-7_6')

Issue #38 (libatomic_ops).

* src/atomic_ops/sysdeps/gcc/aarch64.h [(!__clang__
|| AO_AARCH64_ASM_LOAD_STORE_CAS) && !AO_PREFER_GENERALIZED
&& __ILP32__] (AO_double_load, AO_double_load_acquire, AO_double_store,
AO_double_store_release): Specify the size ("w") of arguments of the
aarch64 instructions.
* src/atomic_ops/sysdeps/gcc/aarch64.h [(!__clang__
|| AO_AARCH64_ASM_LOAD_STORE_CAS) && __ILP32__]
(AO_double_compare_and_swap, AO_double_compare_and_swap_acquire,
AO_double_compare_and_swap_release, AO_double_compare_and_swap_full):
Likewise.
* src/atomic_ops/sysdeps/standard_ao_double_t.h [__aarch64__
&& __ILP32__] (double_ptr_storage): Define as unsigned long long
(instead of unsigned __int128).