Fix writeback for Thumb LdmStm

Former-commit-id: 00a15dc99716290da7b260b261090454d621ce8a

src/arm7tdmi/thumb/exec.rs

@@ -375,6 +375,8 @@ impl Core {
             }
         }
 
+        self.gpr[rb] = addr as u32;
+
         Ok(CpuPipelineAction::IncPC)
     }
 

src/arm7tdmi/thumb/mod.rs

@@ -219,7 +219,10 @@ impl ThumbInstruction {
     }
 
     pub fn rb(&self) -> usize {
-        self.raw.bit_range(3..6) as usize
+        match self.fmt {
+            ThumbFormat::LdmStm => self.raw.bit_range(8..11) as usize,
+            _ => self.raw.bit_range(3..6) as usize,
+        }
     }
 
     pub fn ro(&self) -> usize {