cpu/arm: fix MSR by changing modes

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

src/cpu/arm/exec.cc

@@ -377,6 +377,11 @@ Instruction::exec(Cpu& cpu) {
                     pc_error(data.operand);
 
                     if (cpu.cpsr.mode() != Mode::User) {
+                        if (!data.spsr) {
+                            Psr tmp = Psr(cpu.gpr[data.operand]);
+                            cpu.chg_mode(tmp.mode());
+                        }
+
                         psr.set_all(cpu.gpr[data.operand]);
                     }
                     break;