[PARISC] Arch-specific compat signals

Add enough arch-specific compat signals code to enable parisc64
to compile and boot out of the mainline tree. There are likely still
many dragons here, but this is a start to squashing the last
big difference between the mainline tree and the parisc-linux tree.
The remaining bugs can be squashed as they come up.

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>