Rustdoc: add intra-page navigation to native module pages

[bstrie]

Compare http://doc.rust-lang.org/doc/std/os_libc_constants.html and http://doc.rust-lang.org/doc/std/os_libc.html and note the absence of navigational links at the top of the latter page. Pages for native modules should be treated the same as pages for ordinary modules and be given table of contents-style links at the top of the page for easier summary and navigation.