Remove unsafe emulation-based implementation of double CAS (SunCC/x86)

Remove unsafe emulation-based implementation of double CAS (SunCC/x86)

* src/atomic_ops/sysdeps/sunc/x86.h
(AO_compare_double_and_swap_double_full): Do not test
AO_WEAK_DOUBLE_CAS_EMULATION and remove emulation-based implementation
since it is unsafe (not atomic with respect to other kinds of updates)
leading to test_stack failure.
* configure.ac (AO_WEAK_DOUBLE_CAS_EMULATION): Update and refine
documentation.
* src/atomic_ops/sysdeps/sunc/x86.h
(AO_compare_double_and_swap_double_full): Likewise.