Main Thread Evacuated

[Wolfgang Thaller]

I've found a bug in the scheduler:

When a main thread finishes (e.g. returning from a ffi callback), the 
GC might be run before it is removed from the main_threads list. If a 
major collection happens, the thread will be garbage collected and the 
GC barfs when it updates the pointers in the main_threads list.
This scenario is quite unlikely during normal operation, but when 
heap-profiling a HOpenGL program it leads to certain death after a few 
seconds.
I don't know how to fix it in a clean way (I have no idea how my 
changes would affect the threaded rts or the other "ways") --- so can 
someone else please fix this?

Cheers,

Wolfgang