Revive 1989: Thread detach stability

[MoonlightSentinel]

Rebases #1989. The original PR was already approved but got stalled because of a TLS issue AFAICT,

Let's see whether this issue was resolved since then.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18063	major	thread_attachThis returns dangling pointer

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#2966"

[MoonlightSentinel]

Well, seems like there are some remaining issues