[flang] Lower exit intrinsic

This patch adds lowering for the `exit`
intrinsic.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D121882



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Josh Mottley <Josh.Mottley@arm.com>