[PARISC] unwinder improvements

Add special-case handling for "handle_interruption" so that we can rewind
past the interruption. This is useful for seeing what caused a BUG() or
WARN_ON(); otherwise the unwind stops at the interruption.

Signed-off-by: Randolph Chung <tausq@debian.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>