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

Analog of what commit 494486a1d2697f2153199b6501ab5b4d6e15a2bb 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 28e6103665301ce60634e8a77f0b657c6cc099de (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>