feat(lint): surface the amd64 extension layer's refusals

Assisted-by: GLM 5.3 Flash

lint/registers.go

@@ -71,6 +71,13 @@ func scanNonportableRegisters(t *ast.Text) []Diagnostic {
 		if !ok {
 			continue
 		}
+		// A statement pinned to the extension layer sits outside the
+		// portability premise already: go tool asm knows none of its
+		// mnemonics, so advising the canonical word spelling inside one is
+		// noise on a statement the toolchain would refuse whole.
+		if amd64ExtPinned(strings.ToUpper(in.Mnemonic.Text)) {
+			continue
+		}
 		for _, op := range in.Operands {
 			if op.Addr.Sym != nil && op.Addr.Sym.Pseudo == "" {
 				report(op.Addr.Sym.Name, op.Pos)