target/arm: hide aliased MIDR from gdbstub

This is just a constant alias register with the same value as the
"other" MIDR so it serves no purpose being presented to gdbstub.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20231106185112.2755262-6-alex.bennee@linaro.org>

target/arm/helper.c

@@ -9000,7 +9000,7 @@ void register_cp_regs_for_features(ARMCPU *cpu)
               .type = ARM_CP_CONST, .resetvalue = cpu->revidr },
         };
         ARMCPRegInfo id_v8_midr_alias_cp_reginfo = {
-            .name = "MIDR", .type = ARM_CP_ALIAS | ARM_CP_CONST,
+            .name = "MIDR", .type = ARM_CP_ALIAS | ARM_CP_CONST | ARM_CP_NO_GDB,
             .cp = 15, .crn = 0, .crm = 0, .opc1 = 0, .opc2 = 4,
             .access = PL1_R, .resetvalue = cpu->midr
         };