This is a nice cleanup, arch/powerpc/Makefile is long and messy so
moving this out helps a little.

It also allows us to do:

  $ make arch/powerpc

Which can be helpful if you just want to compile test some changes to
arch code and not link everything.

Finally it also gives us a single place to do subdir-cc-flags
assignments which affect the whole of arch/powerpc, which we will do
in a future patch.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>