Avoid template ambiguities using type_identity_t.

Previously the compiler would be unable to figure out whether

     uint8_t x;
     core_util_atomic_store(&x, 0);

should invoke core_util_atomic_store<uint8_t>, matching the pointer
type, or core_util_atomic_store<int>, matching the value, leading to
an ambiguity error.

Templates now select only on the type of the atomic pointer parameter.