Code refactoring of Clang double-wide intrinsic workarounds (gcc/x86)

Code refactoring of Clang double-wide intrinsic workarounds (gcc/x86)
(fix commits 9803386, f6a1b68, eb6d4fd)

* src/atomic_ops/sysdeps/gcc/generic.h [AO_HAVE_DOUBLE_PTR_STORAGE
&& !AO_HAVE_double_store] (AO_double_store): Do not define if
AO_SKIPATOMIC_double_store.
* src/atomic_ops/sysdeps/gcc/generic.h [AO_HAVE_DOUBLE_PTR_STORAGE
&& !AO_HAVE_double_store_release] (AO_double_store_release): Do not
define if AO_SKIPATOMIC_double_load.
* src/atomic_ops/sysdeps/gcc/x86.h [AO_GCC_ATOMIC_TEST_AND_SET
&& __clang__ && !__x86_64__ && !__APPLE_CC__ && !__CYGWIN__
&& !AO_PREFER_BUILTIN_ATOMICS] (AO_SKIPATOMIC_double_load,
AO_SKIPATOMIC_double_load_acquire, AO_SKIPATOMIC_double_store,
AO_SKIPATOMIC_double_store_release): Define internal macro (undefined
at the end of file).
* src/atomic_ops/sysdeps/gcc/x86.h [AO_GCC_ATOMIC_TEST_AND_SET
&& (__clang__ && !__x86_64__ && !__APPLE_CC__ && !__CYGWIN__
&& !AO_PREFER_BUILTIN_ATOMICS) || (__APPLE_CC__ && __x86_64__))]
(AO_SKIPATOMIC_double_compare_and_swap_ANY): Move definition here
(remove at other places in this file); undefine at the end of the file.
* src/atomic_ops/sysdeps/gcc/x86.h [AO_GCC_ATOMIC_TEST_AND_SET]: Check
AO_SKIPATOMIC_double_compare_and_swap_ANY instead of checking multiple
macros to detect whether a workaround for the CAS (and others)
double-wide operations is needed.