Fix sporadic build failures due to race when running make installworld

Fix sporadic build failures due to race when running make installworld
when strip gets replaced at install time by adding it to ITOOLS for the
default usr.bin/xinstall STRIP_CMD

This will fix the failure noted in this Jenkins build step:
https://jenkins.freebsd.org/job/Build-UFS-image/688/

This will also fix the issue reported by alfred@ dealing with installing on
targets that differ from build hosts (e.g. installing on i386/i386 when built
on amd64/amd64)

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division