kbuild: do not warn about __*init/__*exit symbols being exported

We have several legitimate uses where we export symbols
annotated with one of:
__devinit, __cpuinit, __meminit and their exit counterpart.
So let's stop warning about those being exported in favour
of adding all sorts of workaround to silence the warning.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>