[PARISC] fix signal trampoline cache flushing

The signal trampolines were accidently flushing the kernel I$ instead of
the users.  Fix that up, and also add a missing user D$ flush while
we're at it.

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