Pass -nostdlib -nostartfiles together with -r [BZ #31753]

Since -r in GCC 6/7/8 doesn't imply -nostdlib -nostartfiles, update the
link-static-libc.out rule to also pass -nostdlib -nostartfiles.  This
fixes BZ #31753.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>