powerpc/virtex5: Fix Virtex5 machine check handling

The 440x5 core in the Virtex5 uses 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: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>