powerpc/64s/exception: Add EXC_HV_OR_STD, which selects HSRR if HVMODE

Add EXC_HV_OR_STD and use it to consolidate the 0x500 external
interrupt.

Executed code is unchanged.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-20-npiggin@gmail.com