arch: xtensa: set __start as entry point for Xtensa.

__start is deemed the entry point for all architectures in Zephyr.
Accordingly, Xtensa code had to be modified a bit to fall in line
with this convention.

Change-Id: If3ed344721c9f2735378b866662a68d8d5795324
Signed-off-by: Rajavardhan Gundi <rajavardhan.gundi@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>