Tls does not prevent GC

[Aurel300]

Something I found hidden in the hl docs that didn't show in the API docs:

https://github.com/HaxeFoundation/haxe/blob/72ef1d389e1f1ae3fdbf8a9f6bf76d39774db23b/std/sys/thread/Tls.hx#L32-L33

I'm guessing there should be some hl_add_root calls in thread.c.