Make multiversioning work with internal linkage

We previously made all multiversioning resolvers/ifuncs have weak
ODR linkage in IR, since we NEED to emit the whole resolver every time
we see a call, but it is not necessarily the place where all the
definitions live.

HOWEVER, when doing so, we neglected the case where the versions have
internal linkage.  This patch ensures we do this, so you don't get weird
behavior with static functions.