Remove duplicate AO_spin and AO_pause definition in atomic_ops_stack

Remove duplicate AO_spin and AO_pause definition in atomic_ops_stack

* src/atomic_ops.c (AO_REQUIRE_CAS): Undefine and include atomic_ops.h
unconditionally.
* src/atomic_ops.c (AO_USE_WIN32_PTHREADS): Define macro (and include
windows.h) if Win32 (non-POSIX) to have Sleep-based implementation of
AO_pause.
* src/atomic_ops.c (dummy, AO_spin, AO_pause): Define unconditionally
(to be always available for atomic_ops_stack).
* src/atomic_ops.c (dummy): Rename to spin_dummy.
* src/atomic_ops_stack.c (dummy, AO_spin, AO_pause): Remove definition
(for Win32); remove FIXME.
* src/atomic_ops_stack.c (AO_pause): Declare if
AO_USE_ALMOST_LOCK_FREE defined.