Fix Issue 22443 - OpenBSD: Fix Fiber support by adding MAP_STACK

[ibara]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22443	major	OpenBSD: Fix Fiber support by adding MAP_STACK

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 "stable + druntime#3601"