Force R15 alignment via cpu.set_reg

I've encounted many bugs with instructions that write PC but don't align
it.


Former-commit-id: b6a234564729f83c0b5e8adfa293227299aad4ac

src/arm7tdmi/cpu.rs

@@ -116,7 +116,7 @@ impl Core {
     pub fn set_reg(&mut self, reg_num: usize, val: u32) {
         match reg_num {
             0...14 => self.gpr[reg_num] = val,
-            15 => self.pc = val,
+            15 => self.pc = val & !1,
             _ => panic!("invalid register"),
         }
     }