Workaround missing built-in double-wide primitives for AArch64 in gcc-4.7

* 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): Define using inline assembly
(because GCC 4.7-2013.01 has no support of 16-byte atomic operations).
* 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): Do not define if the corresponding
AO_HAVE_double_X macro is defined.
Use AO_EXPECT_FALSE for AArch64 double_load/store

* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_HAVE_DOUBLE_PTR_STORAGE):
Do not check (since defined in standard_ao_double_t.h for AArch64).
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_double_load): Add TODO
items.
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_double_load,
AO_double_load_acquire, AO_double_store, AO_double_store_release):
Use AO_EXPECT_FALSE.