arch-arm: Decode Brk64 instructions

The brk instruction in aarch64 was decoded as an unimplemented
instruction. Fix that.

Change-Id: I3eb36a016ab56d882426c5cdef3a0b594de0f9cd
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8142
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

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

@@ -254,7 +254,7 @@ namespace Aarch64
                   case 0x03:
                     return new Smc64(machInst);
                   case 0x04:
-                    return new FailUnimplemented("brk", machInst);
+                    return new Brk64(machInst);
                   case 0x08:
                     return new FailUnimplemented("hlt", machInst);
                   case 0x15: