rustc: clarify warning about native deps for a staticlib.

rustc: clarify warning about native deps for a staticlib.

This adjusts the "unlinked native library" warning one receives when
compiling with `crate_type="staticlib"`. The warning is just trying to
tell the user that they need to link against these libraries, but the
old text wasn't making this obvious, the new text says this explicitly.

It also squashes a bug where native dependencies can be listed multiple
times, if several Rust libraries depend on them.