various fixes for naked_asm! implementation

- fix for divergence
- fix error message
- fix another cranelift test
- fix some cranelift things
- don't set the NORETURN option for naked asm
- fix use of naked_asm! in doc comment
- fix use of naked_asm! in run-make test
- use `span_bug` in unreachable branch

example/mini_core_hello_world.rs

@@ -390,7 +390,7 @@ global_asm! {
 #[naked]
 extern "C" fn naked_test() {
     unsafe {
-        asm!("ret", options(noreturn));
+        naked_asm!("ret");
     }
 }