rustc asserts on `extern fn main() {}`

A file with nothing but `extern fn main() {}` gives this:

```
rustc: /media/linhaus/rust/src/llvm/lib/IR/Instructions.cpp:276: void llvm::CallInst::init(llvm::Value*, llvm::ArrayRef<llvm::Value*>, const llvm::Twine&): Assertion `(Args.size() == FTy->getNumParams() || (FTy->isVarArg() && Args.size() > FTy->getNumParams())) && "Calling a function with bad signature!"' failed.
Aborted
```
