Reorder AO_double_t union elements for AO_DOUBLE_T_INITIALIZER portability

Reorder AO_double_t union elements for AO_DOUBLE_T_INITIALIZER portability

* src/atomic_ops/sysdeps/standard_ao_double_t.h (AO_double_t):
Make "AO_parts" element to be the first one in the enum so that
AO_DOUBLE_T_INITIALIZER could be defined both strictly complying with
ANSI C standard (thus preventing "missing braces around initializer"
64-bit compiler warning) and independently of __m128 type definition;
add comment.
* src/atomic_ops/sysdeps/standard_ao_double_t.h
(AO_DOUBLE_T_INITIALIZER): Update to reflect the change in AO_double_t.