powerpc/64s/exception: Expand EXC_COMMON and EXC_COMMON_ASYNC macros

These don't provide a large amount of code sharing. Removing them
makes code easier to shuffle around. For example, some of the common
instructions will be moved into the common code gen macro.

No generated code change.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200225173541.1549955-5-npiggin@gmail.com