kbuild: suppress modpost warnings for references from the .toc section as used by powerpc

We should do better here by effetively "dereferencing" references to
the .toc (or the .got2) section, but that is much harder.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>