arm: Don't implement BXJ on M-profile CPUs

For M-profile CPUs, the BXJ instruction does not exist at all, and
the encoding should always UNDEF. We were accidentally implementing
it to behave like A-profile BXJ; correct the error.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Message-id: 1491844419-12485-2-git-send-email-peter.maydell@linaro.org

target/arm/translate.c

@@ -10485,7 +10485,12 @@ static int disas_thumb2_insn(CPUARMState *env, DisasContext *s, uint16_t insn_hw
                         }
                         break;
                     case 4: /* bxj */
-                        /* Trivial implementation equivalent to bx.  */
+                        /* Trivial implementation equivalent to bx.
+                         * This instruction doesn't exist at all for M-profile.
+                         */
+                        if (arm_dc_feature(s, ARM_FEATURE_M)) {
+                            goto illegal_op;
+                        }
                         tmp = load_reg(s, rn);
                         gen_bx(s, tmp);
                         break;