Back when I added -Werror in commit ba55bd74360e ("powerpc: Add
configurable -Werror for arch/powerpc") I did it by adding it to most
of the arch Makefiles.

At the time we excluded math-emu, because apparently it didn't build
cleanly. But that seems to have been fixed somewhere in the interim.

So move the -Werror addition to the top-level of the arch, this saves
us from repeating it in every Makefile and means we won't forget to
add it to any new sub-dirs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

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>