Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add hacks that I use to test cross-builds (by building on
native and foreign architectures and comparing products).
They eliminate most of the differences caused by different
object directory paths, timestamping, and identification.

(Note WORLDTMP was renamed to ${OBJTREE}${.CURDIR}/tmp.)