powerpc/64s/exception: Move EXCEPTION_COMMON handler and return branches into callers

The aim is to reduce the amount of indirection it takes to get through
the exception handler macros, particularly where it provides little
code sharing.

No generated code change.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>