Fix code on big-endian gcc / clang

Ouch!  We need a bigendian bot...

Fixes https://github.com/harfbuzz/harfbuzz/issues/1498

src/hb-machinery.hh

@@ -682,11 +682,17 @@ struct BEInt<Type, 2>
   }
   operator Type () const
   {
-#if (defined(__GNUC__) && __GNUC__ >= 5) || defined(__clang__)
+#if ((defined(__GNUC__) && __GNUC__ >= 5) || defined(__clang__)) && \
+    defined(__BYTE_ORDER) && \
+    (__BYTE_ORDER == __LITTLE_ENDIAN || __BYTE_ORDER == __BIG_ENDIAN)
     /* Spoon-feed the compiler a big-endian integer with alignment 1.
      * https://github.com/harfbuzz/harfbuzz/pull/1398 */
     struct __attribute__((packed)) packed_uint16_t { uint16_t v; };
+#if __BYTE_ORDER == __LITTLE_ENDIAN
     return __builtin_bswap16 (((packed_uint16_t *) this)->v);
+#else /* __BYTE_ORDER == __BIG_ENDIAN */
+    return ((packed_uint16_t *) this)->v;
+#endif
 #endif
     return (v[0] <<  8)
          + (v[1]      );