[flang] Add ETIME runtime and lowering intrinsics implementation (#90578)

This patch add support of intrinsics GNU extension ETIME
https://github.com/llvm/llvm-project/issues/84205. Some usage info and
example has been added to `flang/docs/Intrinsics.md`. The patch contains
both the lowering and the runtime code and works on both Windows and
Linux.


|   System  |   Implmentation  |
|-----------|--------------------|
| Windows| GetProcessTimes |
| Linux      |times                     |