Add #![no_main] attribute and Linux entry point

src/main.rs

@@ -1,8 +1,12 @@
 #![no_std]
+#![no_main]
 
 use core::panic::PanicInfo;
 
-fn main() {}
+#[no_mangle]
+pub extern "C" fn _start() -> ! {
+    loop {}
+}
 
 /// This function is called on panic.
 #[panic_handler]