gccrs#4557: Add lang item exchange_malloc

Link: Rust-GCC/gccrs#4557

#![allow(unused)]
fn main() {
#[lang = "exchange_malloc"]
unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
    let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
    match Global.alloc(layout) {
        Ok(ptr) => ptr.as_mut_ptr(),
        Err(_) => handle_alloc_error(layout),
    }
}
}

Objective

This PR introduces the exchange_malloc lang item to the compiler.

Changes Made

We just need to produce it as a symbol. Then, when we look for it, we need to be able to find it.

Why is this needed for alloc?

This lang item is a strict prerequisite for the owned_box lang item and box expressions.

Test Case

#![allow(unused)]
fn main() {
#[lang = "exchange_malloc"]
unsafe fn _allocate(_size: usize, _align: usize) -> *mut u8 {
    0 as *mut u8
}
}