package/protobuf: fix build failure due to missing -mcmodel=large

package/protobuf: fix build failure due to missing -mcmodel=large

When building protobuf for or1k -mcmodel=large is needed to link, so let's
add that gcc option in case we're building for or1k.

Upstream gcc doesn't have the -mcmodel=large option for or1k, but all
released Buildroot gcc versions have the patch to add it, so that's
fine.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>