LeadByte bitfield order is wrong on big endian platforms
LeadByte bitfield order is wrong on big endian platforms

Use `__BIG_ENDIAN` macro to specify bit_field order. This macro may not be available outside of GCC

Ref: https://groups.google.com/d/msg/gnu.gcc.help/dHkYZVr_pcA/-q0Z0zHtbsEJ
agronholm/cbor2,agronholm/cbor2,agronholm/cbor2