xtensa: move byte-order macros out of arch.h

These are macros that are expected to be defined at all times by
the compiler. We need them at the very beginning of kernel.h for
the k_thread definition, before it's possible to include arch.h.

Make a special toolchain header for XCC compiler and place these
defines in there. Otherwise inherit all the other GCC defines.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>