change... This eliminates a cast, and also forces td_retval
(often 2 32-bit registers) to be aligned so that off_t's can be
stored there on arches with strict alignment requirements like
armeb (AVILA)... On i386, this doesn't change alignment, and on
amd64 it doesn't either, as register_t is already 64bits...
This will also prevent future breakage due to people adding additional
fields to the struct...
This gets AVILA booting a bit farther...
Reviewed by: bde