sparc: keep calling do_signal() as long as pending signals remain

Analog of what commit 494486a1 had done
to alpha (another architecture with similar bug).

One note: in rtrap_32.S part clr %l6 has been a rudiment of left after
commit 28e61036 (sparc: Fix debugger syscall
restart interactions) has killed %l6 use in there.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>