This will allow it to be used as a drop in replacement for
_mesa_little_endian in a number of cases.

v2: - Always define PIPE_ARCH_LITTLE_ENDIAN and PIPE_ARCH_BIG_ENDIAN,
      define the one that reflects the host system to 1 and the other to 0
    - replace all uses of #ifdef, #ifndef, and #if defined() with #if
      and #if ! with PIPE_ARCH_*_ENDIAN

Reviewed-by: Eric Engestrom <eric@engestrom.ch>