Fix 'unknown attribute no_sanitize' compiler warning (clang prior to v3.8)

Fix 'unknown attribute no_sanitize' compiler warning (clang prior to v3.8)

* src/atomic_ops.h [!AO_ATTR_NO_SANITIZE_MEMORY && AO_MEMORY_SANITIZER]
(AO_ATTR_NO_SANITIZE_MEMORY): Do not define to no_sanitize attribute
unless clang-3.8+ (or not clang).
* tests/test_atomic_include.template [AO_HAVE_storeXX
&& AO_MEMORY_SANITIZER && AO_PREFER_GENERALIZED] (test_atomicXX):
Initialize x local variable (before AO_storeXX which can be defined
using CAS); add comment.
* tests/test_atomic_include.template [AO_HAVE_short_storeXX
&& AO_MEMORY_SANITIZER && AO_PREFER_GENERALIZED] (test_atomicXX):
Initialize s local variable.
* tests/test_atomic_include.template [AO_HAVE_char_storeXX
&& AO_MEMORY_SANITIZER && AO_PREFER_GENERALIZED] (test_atomicXX):
Initialize b local variable.
* tests/test_atomic_include.template [AO_HAVE_int_storeXX
&& AO_MEMORY_SANITIZER && AO_PREFER_GENERALIZED] (test_atomicXX):
Initialize zz local variable.