powerpc/64s/exception: Replace PROLOG macros and EXC helpers with a gas macro

This creates a single macro that generates the exception prolog code,
with variants specified by arguments, rather than assorted nested
macros for different variants.

The increasing length of macro argument list is not nice to read or
modify, but this is a temporary condition that will be improved in
later changes.

No generated code change except BUG line number constants and label
names.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-23-npiggin@gmail.com