sh: Fix restartable syscall arg5 clobbering.

We use R0 as the 5th argument of syscall.  When the syscall restarts
after signal handling, we should restore the old value of R0.
The attached patch does it. Without this patch, I've experienced random
failures in the situation which signals are issued frequently.

Signed-off-by: Kaz Kojima <kkojima@rr.iij4u.or.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>