tools/rcutorture: Fix position of -lgcc in mkinitrd.sh

The -lgcc command-line argument is placed poorly in the build options,
which can result in build failures, for exapmle, on ARM when uidiv()
is required.  This commit therefore places the -lgcc argument after the
source files.

Fixes: b94ec368 ("rcutorture: Make use of nolibc when available")
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>