doc: Broken link in "No stdlib" Page.

https://github.com/rust-lang/rust/blob/master/src/libstd/sys/common/unwind/gcc.rs
in the last paragraph is broken.

[steveklabnik]

Thanks. Given that it's an unstable page, this will happen from time to time.

[steveklabnik]

This was actually fixed at some point; the fix is currently in beta, so will be part of Rust 1.11. Thanks!