Use rust ctor crate on no_std when producing an ELF

nostd_rust_ctor.rs

#![no_std]
#![no_main]

extern crate libc;

#[ctor::ctor]
fn foo() {
    unsafe { libc::puts("foo()\x00".as_ptr() as *const i8); }
}

#[ctor::ctor]
fn bar() {
    unsafe { libc::puts("bar()\x00".as_ptr() as *const i8); }
}


fn main() {
    unsafe { libc::puts("main()\x00".as_ptr() as *const i8); }
}

extern "C" {
    #[no_mangle]
    static mut __init_array_start: [Option<unsafe extern "C" fn() -> ()>; 0];
    #[no_mangle]
    static mut __init_array_end: [Option<unsafe extern "C" fn() -> ()>; 0];
}

pub unsafe fn __rustctor_init_array() {
    let count = __init_array_end.as_mut_ptr().offset_from(__init_array_start.as_mut_ptr()) as usize;
    for i in 0..count {
        (*__init_array_start.as_mut_ptr().offset(i as isize)).expect("non-null function pointer")();
    }
}

#[no_mangle]
pub extern "C" fn _start() -> ! {
    unsafe { __rustctor_init_array(); }
    main();
    loop {}
}

#[panic_handler]
fn my_panic(_info: &core::panic::PanicInfo) -> ! {
    loop {}
}