Rare startup crash when using multiple threads

Since a long time I experience a very rare startup crash in GCFreeZone (see also my confusion in #758). When it happens, it only happens rarely, preferably in release builds and when it started happening more often a small sleep or shifting things around helped. But tonight I could create a situation which caused the issue very reliably, I tried to use that to finally find the cause... I'm still not 100% sure but I think the problem is caused by setting gMultiThreadMode to true at the exact wrong moment, namely after entering and before leaving a gc-free-zone. I fixed it for Kha by just setting it to true from the start (Kha always runs mutltithreaded anyway) and so far things seem to be fine for me now.