kernel: ignore z_fatal_halt() from code coverage

The reason we decide to ignore it in code coverage:
1.No test case can cover the function for code coverage.
2.Even if we added a test for testing, it would be marked as
  "never be called by other code" because the function cause
  CPU halted and it can't return.

Signed-off-by: Peng Su <peng.su@intel.com>