Add #[start] attribute to define a new entry point function
auto merge of #5809 : Aatch/rust/start-attr, r=graydon

This implements #5158. Currently it takes the command line args and the crate map. Since it doesn't take a `main` function pointer, you can't actually start the runtime easily, but that seems to be a shim to allow the current `rust_start` function to call into main.

However, you can do an end-run round the io library and do this:

```rust
use core::libc::{write, c_int, c_void, size_t, STDOUT_FILENO};

#[start]
fn my_start(_argc:int, _argv: **u8, _crate_map: *u8) -> int {
    do str::as_buf("Hello World!\n") |s,len| {
        unsafe {
            write(STDOUT_FILENO, s as *c_void, len as size_t);
        }
    }
    return 0;
}
```

Which is the most basic "Hello World" you can do in rust without starting up the runtime (though that has quite a lot to do with the fact that `core::io` uses `@` everywhere...)