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

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -726,6 +726,12 @@ pub macro global_asm() {
     /* compiler built-in */
 }
 
+#[rustc_builtin_macro]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro naked_asm() {
+    /* compiler built-in */
+}
+
 pub static A_STATIC: u8 = 42;
 
 #[lang = "panic_location"]

compiler/rustc_codegen_cranelift/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");
     }
 }