parisc: fix tracing of signals

Mike Frysinger pointed out that calling tracehook_signal_handler with
stepping=0 missed testing the thread flags, resulting in not calling
ptrace_notify. Fix this by testing if we're single stepping or branch
stepping and setting the flag accordingly.

Tested, seems to work.

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>