Store main thread in Jvm.init()

[kevinresol]

Edit: this actually does not fix my event loop problem. We actually need to probe the haxe.EventLoop class in the main function in order to capture the correct "main eventloop" into the TLS....

I made a temp patch lib here: https://github.com/kevinresol/haxe12985/blob/main/src/haxe/issue12985/Patch.hx

A quick fix that is probably not ideal as it still brings in 3 extra classes (w/ full dce) into the output jar for a minimal trace() Haxe app. But they are quite minimal (see below) and I think we should have it.

A more proper fix is probably to store only the native java thread ref, and update sys.thread.Thread to use it.

See #12985

[kevinresol]

I actually tried to follow up by implementing the suggested alternative above. But that won't really fly because the __init__ function is really assuming itself to be run in the main thread (see below)

haxe/std/sys/thread/Thread.hx

Line 313 in 5f83789

currentTLS.value = mainThread;