64e non-maskable interrupts save the state of the irq soft-mask in
asm. This can be done in C in interrupt wrappers as 64s does.

I haven't been able to test this with qemu because it doesn't seem
to cause FSL bookE WDT interrupts.

This makes WatchdogException an NMI interrupt, which affects 32-bit
as well (okay, or create a new handler?)
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210316104206.407354-6-npiggin@gmail.com