Don't run cc --version during cleandir/obj stages

This will no work when there is no cc in $PATH (which is the case before the
cross-tools stage once we no longer inherit $PATH in $WMAKE).
The variables set by bsd.compiler.mk/bsd.linker.mk are not needed in these
stages so this avoids a little bit of makefile parsing.

Reviewed By: emaste
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D16814