Improve 'linker-D-flag' detection for ldmd2

[kinke]

ldmd2 supports all ldc2 ones (and e.g. -link-defaultlib-shared is rather important).

Making these helper functions public will come in handy for reggae as well.

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

[PetarKirov]

LGTM. I suppose this is not changelog-worthy as it's only beneficial for using dub as a library, rather than for end-users.

[PetarKirov]

@kinke I suppose you guys can use the git-paths feature to depend on dub @ 24ca22d in reggae.

[kinke]

I suppose this is not changelog-worthy as it's only beneficial for using dub as a library, rather than for end-users.

The first commit, yes, the 2nd benefits normal usage with ldmd2 as well. I consider that more as a bug fix though, hence no changelog entry.