druntime: remove libunwind dependency and make execinfo optional (LDC upstream)

[JohanEngelen]

Based on:

• Fix libunwind backtraces & Make defaulttraceinfo more generic. ldc-developers/ldc#4639
• Remove druntime's libunwind dependency ldc-developers/ldc#4691

This removes the dependency on libunwind and makes DefaultTraceInfo smart enough to not strictly require execinfo.

Tested to work well with LDC and with DMD here: #20741. (this PR is pre-requisite of #20741)

[dlang-bot]

Thanks for your pull request and interest in making D better, @JohanEngelen! 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#20758"