[POWERPC] Support nested cpu feature sections

This patch adds some macros that can be used with an explicit label in
order to nest cpu features. This should be used very careful but is
necessary for the upcoming cell TB fixup.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>