powerpc: interrupt handler wrapper functions

Add wrapper functions (derived from x86 macros) for interrupt handler
functions. This allows interrupt entry code to be written in C.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-27-npiggin@gmail.com