powerpc/64e/interrupt: Use new interrupt context tracking scheme

With the new interrupt exit code, context tracking can be managed
more precisely, so remove the last of the 64e workarounds and switch
to the new context tracking code already used by 64s.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210316104206.407354-8-npiggin@gmail.com