powerpc/4xx: Fix erroneous xmon warning on PowerPC 4xx

The xmon code relies on MSR_RI being non-zero to indicate that an exception
is recoverable.  If it is not, it prints a warning message.  However, the
PowerPC 4xx cores do not have an MSR_RI bit and this warning is produced for
every xmon event.

This introduces an unrecoverable_excp function to determine if an exception
is recoverable or not.  This gets rid of the erroneous warnings on 4xx.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>