toolchain: Add __BYTE_ORDER__ definition to the GCC toolchain header.

__BYTE_ORDER__ preprocessor definition is not defined by older versions
of GCC. The definitions for __ORDER_BIG_ENDIAN__,
__ORDER_LITTLE_ENDIAN__ and __BYTE_ORDER__ by automatic detection using
arch-specific endianness definitions have been added.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>