powerpc/64s/interrupt: move early boot ILE fixup into a macro

In preparation for using this sequence in machine check interrupt, move
it into a macro, with a small change to make it position independent.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220926055620.2676869-2-npiggin@gmail.com