tilepro: work around module link error with gcc 4.7

tilepro: work around module link error with gcc 4.7

commit 3cb3f839d306443f3d1e79b0bde1a2ad2c12b555 upstream.

gcc 4.7.x is emitting calls to __ffsdi2 where previously
it used to inline the appropriate ctz instructions.
While this needs to be fixed in gcc, it's also easy to avoid
having it cause build failures when building with those
compilers by exporting __ffsdi2 to modules.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>