rcutorture: Add cross-compile capability to initrd.sh

This adds the CROSS_COMPILE environment to the initrd.sh script's
gcc command to enable cross compilation.
Reported-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/bin/mkinitrd.sh

@@ -113,7 +113,7 @@ int main(int argc, int argv[])
 	return 0;
 }
 ___EOF___
-cc -static -Os -o init init.c
+${CROSS_COMPILE}gcc -static -Os -o init init.c
 strip init
 rm init.c
 echo "Done creating a statically linked C-language initrd"