Remove linking and intrinsics code made dead by only supporting LLVM 3.7 and up
This is mostly based on Alex's throwaway comment:
> probably reject those that LLVM just doesn't support...
So I'm more than happy to adjust the PR based on how you thought this should look. Also happy to split it into two PRs, one for linking and one for intrinsics.
r? @alexcrichton
/cc @nagisa @brson