Support linking crates in JIT

[z0w0]

LLVM's dynamic library API is giving hassles, so it might come down to using libuv's dynamic library API. @brson said it would be OK to use libuv, but preferably try and get LLVM's API working first.

The code that should work is commented out in rustc/back/link.rs and RustWrapper.cpp if anyone wants to help debug what's going wrong.

[brson]

Does the version of libuv we currently have in the tree have the dynamic linking features you need, or do we need to upgrade? If we use libuv, let's expose the dynamic linking bindings in std, then use them in rustc (vs linking rustllvm to uv).

[z0w0]

Although it turns out that the issue wasn't with LLVM's API but rather libdl, I'd suggest bumping up libuv, because the version in the tree doesn't support getting an error message (so if you wanted to have an API around it in std, it couldn't have error messages).

[z0w0]

I recently got a new x86_64 computer (my old laptop was x86). Seems to link crates fine now with this commit, but I'm getting an assertion about x86_64 addresses. Not sure if it's related, but I'll look into it.

[brson]

This is the assert it's tripping: http://llvm.org/docs/doxygen/html/RuntimeDyldELF_8cpp_source.html#l00221
And this is the stack trace: http://pastebin.com/raw.php?i=MUEJfx5a

Thanks to z0w0