Workaround MSan warning about uninitialized data read by generalized store

Workaround MSan warning about uninitialized data read by generalized store

* src/atomic_ops.h [__has_feature(memory_sanitizer)]
(AO_MEMORY_SANITIZER): Define.
* src/atomic_ops.h [AO_MEMORY_SANITIZER] (AO_ATTR_NO_SANITIZE_MEMORY):
Likewise.
* src/atomic_ops/generalize-small.h: Regenerate.
* src/atomic_ops/generalize-small.template
[AO_HAVE_XSIZE_compare_and_swap_write && !AO_HAVE_XSIZE_store_write]
(AO_XSIZE_store_write): Add AO_ATTR_NO_SANITIZE_MEMORY attribute.
* src/atomic_ops/generalize-small.template
[AO_HAVE_XSIZE_compare_and_swap && !AO_HAVE_XSIZE_store]
(AO_XSIZE_store): Likewise.
* src/atomic_ops/generalize-small.template
[AO_HAVE_XSIZE_compare_and_swap_release && !AO_HAVE_XSIZE_store_release]
(AO_XSIZE_store_release): Likewise.
* src/atomic_ops/generalize-small.template
[AO_HAVE_XSIZE_compare_and_swap_full && !AO_HAVE_XSIZE_store_full]
(AO_XSIZE_store_full): Likewise.