Make compare_double_and_swap_double based on double_compare_and_swap

Make compare_double_and_swap_double based on double_compare_and_swap
(gcc/arm, msftc/x86)

* src/atomic_ops/sysdeps/gcc/arm.h
(AO_compare_double_and_swap_double): Replace with new
AO_double_compare_and_swap (with old/new_val arguments of AO_double_t
type).
* src/atomic_ops/sysdeps/gcc/arm.h (AO_HAVE_double_compare_and_swap):
New macro.
* src/atomic_ops/sysdeps/gcc/arm.h
(AO_compare_double_and_swap_double): Convert arguments and invoke
AO_double_compare_and_swap; add TODO item.
* src/atomic_ops/sysdeps/msftc/x86.h
(AO_compare_double_and_swap_double_full): Invoke
AO_double_compare_and_swap_full (instead of calling
_InterlockedCompareExchange64 directly); add TODO item.