add mips64-gnu and mips64el-gnu targets

add mips64-gnu and mips64el-gnu targets

With this commit one can build no_core (and probably no_std as well)
Rust programs for these targets. It's not yet possible to cross compile
std for these targets because rust-lang/libc doesn't know about the
mips64 architecture.

These targets have been tested by cross compiling the "smallest hello"
program (see code below) and then running it under QEMU.

``` rust
#![feature(start)]
#![feature(lang_items)]
#![feature(no_core)]
#![no_core]

#[link(name = "c")]
extern {
    fn puts(_: *const u8);
}

#[start]
fn start(_: isize, _: *const *const u8) -> isize {
    unsafe {
        let msg = b"Hello, world!\0";
        puts(msg as *const _ as *const u8);
    }
    0
}

#[lang = "copy"]
trait Copy {}

#[lang = "sized"]
trait Sized {}
```