powerpc: Add logic to patch alternative feature sections

This commit adds the logic to patch alternative sections.  This is fairly
straightforward, except for branches.  Relative branches that jump from
inside the else section to outside of it need to be translated as they're
moved, otherwise they will jump to the wrong location.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>