powerpc/booke64: Fix machine check handler to use the right prolog

Machine check exception handler was using a wrong prolog. Hypervisors like
KVM which are called early from the exception handler rely on the interrupt
source.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>