arch-arm: Change disassemble when MSR to UNKNOWN register

This patch changes the fault being thrown when MSR/MRS to an unknown
Misc register in AArch64. While previously the instruction was decoded
as an Unknown instruction (hence not printing any information), it is
now decoded as a FailUnimplemented and the unrecognized System register
numbers (CRn, op0...) are printed.

Change-Id: I205ff7adcde5934231c77e8d2250db69a34581fc
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10061
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/arch/arm/isa/formats/aarch64.isa

@@ -369,7 +369,14 @@ namespace Aarch64
                     }
                     // Check for invalid registers
                     if (miscReg == MISCREG_UNKNOWN) {
-                        return new Unknown64(machInst);
+                        auto full_mnemonic =
+                            csprintf("%s op0:%d op1:%d crn:%d crm:%d op2:%d",
+                                     read ? "mrs" : "msr",
+                                     op0, op1, crn, crm, op2);
+
+                        return new FailUnimplemented(read ? "mrs" : "msr",
+                            machInst, full_mnemonic);
+
                     } else if (miscRegInfo[miscReg][MISCREG_IMPLEMENTED]) {
                         if (miscReg == MISCREG_NZCV) {
                             if (read)