fairly straightforward:

        #[global_allocator]
        static GLOBAL: StupidAlloc = StupidAlloc;
        
        #[no_mangle]
        pub unsafe extern "C" fn stupid_malloc(size: usize) -> *mut u8 {
            let layout = Layout::from_size_align(size, 1).unwrap();
            GLOBAL.alloc(layout)
        }
        
        #[no_mangle]
        pub unsafe extern "C" fn stupid_free(ptr: *mut u8) {
            let layout = Layout::from_size_align(1, 1).unwrap();
            GLOBAL.dealloc(ptr, layout)
        }

then build with --crate-type cdylib, giving you a .so you can link