powerpc: Reduce scariness of interrupt frames in stack traces

Some people see things like "Exception: 501" in stack traces in dmesg
and assume that means that something has gone badly wrong, when in
fact "Exception: 501" just means a device interrupt was taken.
This changes "Exception" to "interrupt" to make it clearer that we
are just recording the fact of a change in control flow rather than
some error condition.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>