[flang] Add GETCWD runtime and lowering intrinsics implementation (#92746)

This patch add support of intrinsics GNU extension GETCWD
https://github.com/llvm/llvm-project/issues/84203. 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 | _getcwd               |
| Linux       |getcwd                  |