message: Improve endian detection.
  Add support for using __BYTE_ORDER__, to support platforms where g++
  doesn't explicitly define __BIG_ENDIAN__, like Solaris Sparc.
diff: #endif // __s390x__
  #if !defined(FLATBUFFERS_LITTLEENDIAN)
  #if defined(__GNUC__) || defined(__clang__)
  - #ifdef __BIG_ENDIAN__
  + #if (defined(__BIG_ENDIAN__) || \
  + (defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__))
  #define FLATBUFFERS_LITTLEENDIAN 0
  #else
  #define FLATBUFFERS_LITTLEENDIAN 1
  