powerpc/44x: Fix 460EX/460GT machine check handling

Those cores use the 440A type machine check (ie, they have
MCSRR0/MCSRR1). They thus need to call the appropriate fixup
function to hook the right variant of the exception.

Without this, all machine checks become fatal due to loss
of context when entering the exception handler.

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