fix(arm64): reject bare BTI with a diagnostic and accept the full family

Assisted-by: GLM 5.3 Flash

asm/arm64_assemble.go

@@ -2856,8 +2856,14 @@ func encodeARM64Sys(mnem string, ops []*ast.Operand) ([]byte, error) {
 		}
 		return a64wordLE(0xd503201f | uint32(v)<<5), nil
 	case "BTI":
+		// The toolchain requires the landing-pad kind: bare BTI is
+		// rejected ("missing operand"), and only the uppercase C/J/JC
+		// spellings assemble (0xd503245f/49f/4df).
+		if len(ops) != 1 {
+			return nil, fmt.Errorf("%s expects C, J, or JC", mnem)
+		}
 		op := operandRegName(ops[0])
-		base, ok := map[string]uint32{"C": 0xd503245f}[op]
+		base, ok := map[string]uint32{"C": 0xd503245f, "J": 0xd503249f, "JC": 0xd50324df}[op]
 		if !ok {
 			return nil, fmt.Errorf("%s: unknown kind %q", mnem, op)
 		}