powerpc: define FUNC_START/FUNC_END

gcc provides FUNC_START/FUNC_END macros to help with creating
assembly functions. Mirror these in the kernel so we can more easily
share code between userspace and the kernel. FUNC_END is just a
stub since we don't currently annotate the end of kernel functions.

It might make sense to do a wholesale search and replace, but for
now just create a couple of defines.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>