Remove improper comment for AO_XSIZE_load_read

* src/atomic_ops/generalize-small.template (AO_XSIZE_load_read):
Remove improper comment (because nop barrier type matches that of
load primitive).
* src/atomic_ops/generalize-small.template (AO_XSIZE_store_write):
Reorder AO_HAVE_nop_write and AO_HAVE_XSIZE_store checking (to match
that for other primitives in this file).
* src/atomic_ops/generalize-small.h: Regenerate.