Add #[start] attribute to define a new entry point function

[Aatch]

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:

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...)

[Aatch]

Uh, the windows build failed for reasons that I'm fairly sure are unrelated to this commit.

[brson]

Since your test case does not have a main function it needs to contain an // xfail-fast line near the top. This will prevent the windows-specific test driver from trying to compile it and call main.