powerpc/64/syscall: Reconcile interrupts

This reconciles interrupts in the system call case like all other
interrupts. This allows system_call_common to be shared with the scv
system call implementation in a subsequent patch.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200225173541.1549955-31-npiggin@gmail.com