powerpc/64e/interrupt: always save nvgprs on interrupt

In order to use the C interrupt return, nvgprs must always be saved.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210316104206.407354-3-npiggin@gmail.com