Fix #22702 - make compilant with getLinkage returning System or Obj-C

[adamdruppe]

druntime uses getLinkage inappropriately in a couple places. The spec says it can return System so we should treat that the same as C/Windows where appropriate, and Throwable should only be a D class, not just not Windows/C++ class - remember, there's Objective-C classes too. The fix is to check what you expect, not a hardcoded list.

[dlang-bot]

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

Auto-close	Bugzilla	Severity	Description
✓	22702	minor	druntime not compliant with D spec re getLinkage

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 + druntime#3705"

[adamdruppe]

The omf failure it gives is fixed in another PR already open; it was something that slipped through the test to master. Once that's merged this should magically fix itself.

[thewilsonator]

Once that's merged this should magically fix itself.

You'll still need to rebase.