buildgcc: Fix msys2 crossgcc build fail

A leading double slash can result when $DESTDIR/$TARGETDIR is expanded
in the libelf portion of buildgcc. The leading double slash causes buildgcc to fail when run from Windows/Msys2. Replace $DESTDIR/$TARGETDIR
with $DESTDIR$TARGETDIR to avoid the problem.

Change-Id: Ide2bae41c07c1566f80104c3a2e2acab53de0d17
Signed-off-by: Scott Duplichan <scott@notabs.org>
Reviewed-on: http://review.coreboot.org/7788
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>