Fix generalized store_release_write for double-wide type on x64 (GCC)

* src/atomic_ops/generalize-small.template
(AO_XSIZE_store_release_write): Move store-write-based definition down
to follow AO_XSIZE_store_write one (since AO_double_store is currently
defined via CAS for gcc/x64).
* src/atomic_ops/generalize-small.h: Regenerate.