native_lib_path attribute

[brson]

Add a crate attribute, #[native_lib_path = ""] to act as an official, possibly smarter, version of #[link_args = "-L something"]

[brson]

Actually, since I'm planning (#2197) on making -L apply to both crates and native libs, maybe for now we should just add #[addl_lib_path] to add both to the crate and native paths.

[Aatch]

I'm pretty sure this isn't valid any more.

Re-open if you disagree.