Properly fix compare_double_and_swap_double for gcc/x32 (use built-in CAS)

Properly fix compare_double_and_swap_double for gcc/x32 (use built-in CAS)

* src/atomic_ops/sysdeps/gcc/x86.h (AO_double_compare_and_swap_full):
New function (only if __x86_64__) implemented using GCC built-in
__sync CAS primitive available for x32.
* src/atomic_ops/sysdeps/gcc/x86.h
(AO_HAVE_double_compare_and_swap_full): New macro (for x32 only).
* src/atomic_ops/sysdeps/gcc/x86.h
(AO_compare_double_and_swap_double_full): Implement using
double_compare_and_swap_full instead of cmpxchg8b for x32 (since
x86_64 has available cmpxchg and cmpxchg16b but not cmpxchg8b).