powerpc: modules: comment about de-dotifying symbols when using the ELFv2 ABI.

ELFv2 doesn't use function descriptors, so we don't expect symbols to
start with ".".  But because depmod and modpost strip ".", and we have
the special symbol ".TOC.", we still need to do it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>