add an endianness meta attribute

[thestinger]

Rust currently doesn't run on a big-endian arch, but it would be nice to be able to write forward-compatible code when dealing with low-level details in bindings, since at some point it likely will. AFAICT this would involve exposing a constant in the libc module (how libc++ does it) and then adding it to the src/librustc/driver/driver.rs.