fix: replace accidental removal of code in Cpu.ChangeMode

src/cpu.zig

@@ -247,6 +247,8 @@ pub const Arm7tdmi = struct {
                 self.spsr = self.banked_spsr[bankedSpsrIndex(next)];
             },
         }
+
+        self.cpsr.mode.write(@enumToInt(next));
     }
 
     pub fn fastBoot(self: *Self) void {