[SPARC64]: Fix ptrace/strace

Don't clobber register %l0 while checking TI_SYS_NOERROR value in
syscall return path.  This bug was introduced by:

db7d9a4e



Problem narrowed down by Luis F. Ortiz and Richard Mortimer.

I tried using %l2 as suggested by Luis and that works for me.

Looking at the code I wonder if it makes sense to simplify the code
a little bit. The following works for me but I'm not sure how to
exercise the "NOERROR" codepath.

Signed-off-by: David S. Miller <davem@davemloft.net>