powerpc/64s: Exception macro for stack frame and initial register save

This code is common to a few exceptions, and another user will be added.
This causes a trivial change to generated code:

-     604: std     r9,416(r1)
-     608: mfspr   r11,314
-     60c: std     r11,368(r1)
-     610: mfspr   r12,315
+     604: mfspr   r11,314
+     608: mfspr   r12,315
+     60c: std     r9,416(r1)
+     610: std     r11,368(r1)

machine_check_powernv_early could also use this, but that requires non
trivial changes to generated code, so that's for another patch.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>