Rollup merge of #119365 - nbdd0121:asm-goto, r=Amanieu

Add asm goto support to `asm!`

Tracking issue: #119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``

compiler/rustc_builtin_macros/src/errors.rs

@@ -766,6 +766,13 @@ pub(crate) struct AsmNoReturn {
     pub(crate) outputs_sp: Vec<Span>,
 }
 
+#[derive(Diagnostic)]
+#[diag(builtin_macros_asm_mayunwind)]
+pub(crate) struct AsmMayUnwind {
+    #[primary_span]
+    pub(crate) labels_sp: Vec<Span>,
+}
+
 #[derive(Diagnostic)]
 #[diag(builtin_macros_global_asm_clobber_abi)]
 pub(crate) struct GlobalAsmClobberAbi {