soc/intel_adsp: Replace Zephyr assembly entry stub

The Zephyr symbols are now part of the same link as the bootloader, so
no need to have an assembly entry stub or fixed address at all.  Just
call z_cstart() as a normally-relocated function.  Interestingly
Zephyr never put a declaration for it in public headers, because this
appears to be the first platform calling it from C.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>