[flang] Lower system_clock intrinsic

This patch adds lowering ofr the `system_clock` intrinsic.
The call is lowered to runtime function call.

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

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D121776



Co-authored-by: V Donaldson <vdonaldson@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>