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)

Sponsored by: EMC / Isilon Storage Division