Use __BYTE_ORDER__ instead of BYTE_ORDER
Use __BYTE_ORDER__ instead of BYTE_ORDER

I'm not sure why the old one stopped working, but the MIPS version of
this file has always been using __BYTE_ORDER__, and that is defined.
I also had to use __ORDER_LITTLE_ENDIAN__ instead of LITTLE_ENDIAN, as
that has gone away as well.

manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain