Always accept .o and .obj files extensions from CLI for all platforms

[denizzzka]

Proposal to discuss
Explanation: ldc-developers/ldc#4733

This patch makes DMD behave the same as clang and maybe all other modern compilers. Seems, there is no point in discarding *.o or *.obj depending on current OS - both extensions point us that this is object file

[dlang-bot]

Thanks for your pull request and interest in making D better, @denizzzka! 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.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#20609"

[rikkimax]

Don't forget to make this change in dub as well, it does filtering.

[denizzzka]

@rikkimax It seems like no changes are needed to DUB: this PR only affects reading of external object files provided by 3rd party build systems (like CMake). If current DUb does things correct it will not need to fix anything

[dkorpel]

The idea looks okay to me, I just have a comment about the code quality.