powerpc: Add -Werror at arch/powerpc level

Back when I added -Werror in commit ba55bd7436 ("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>

arch/powerpc/Kbuild

@@ -1,3 +1,5 @@
+subdir-ccflags-$(CONFIG_PPC_WERROR) := -Werror
+
 obj-y += kernel/
 obj-y += mm/
 obj-y += lib/