Fix bitfields on big-endian machines.

Fixes rust-lang#1340

src/codegen/bitfield_unit.rs

@@ -27,7 +27,12 @@ where
         let byte_index = index / 8;
         let byte = self.storage.as_ref()[byte_index];
 
-        let bit_index = index % 8;
+        let mut bit_index = index % 8;
+        #[cfg(target_endian = "big")]
+        {
+            // Adjust the index for endianness.
+            bit_index = 7 - bit_index;
+        }
         let mask = 1 << bit_index;
 
         byte & mask == mask
@@ -40,7 +45,12 @@ where
         let byte_index = index / 8;
         let byte = &mut self.storage.as_mut()[byte_index];
 
-        let bit_index = index % 8;
+        let mut bit_index = index % 8;
+        #[cfg(target_endian = "big")]
+        {
+            // Adjust the index for endianness.
+            bit_index = 7 - bit_index;
+        }
         let mask = 1 << bit_index;
 
         if val {
@@ -60,7 +70,13 @@ where
 
         for i in 0..(bit_width as usize) {
             if self.get_bit(i + bit_offset) {
-                val |= 1 << i;
+                let mut index  = i;
+                #[cfg(target_endian = "big")]
+                {
+                   // Adjust the index for endianness.
+                   index  = bit_width as usize - 1 - index;
+                }
+                val |= 1 << index;
             }
         }
 
@@ -76,7 +92,13 @@ where
         for i in 0..(bit_width as usize) {
             let mask = 1 << i;
             let val_bit_is_set = val & mask == mask;
-            self.set_bit(i + bit_offset, val_bit_is_set);
+            let mut index  = i;
+            #[cfg(target_endian = "big")]
+            {
+                // Adjust the index for endianness.
+                index  = bit_width as usize - 1 - index;
+            }
+            self.set_bit(index + bit_offset, val_bit_is_set);
         }
     }
 }