kernel: add CODE_UNREACHABLE in _StackCheckHandler

* _StackCheckHandler is FUNC_NORETURN
* if _ARCH_EXCPET is redefined for specific arch and
  has function return in some cases, e.g., interrupt or
  exception, a compiler warning will come out
* So add CODE_UNREACHABLE to guarantee it will not return

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>