powerpc/64s: move NMI soft-mask handling to C

Saving and restoring soft-mask state can now be done in C using the
interrupt handler wrapper functions.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-41-npiggin@gmail.com