Anonymous mapped regions increases unlimitely on spawn

[unDEFER]

So in digging by this problem, I have made simple patch to 
druntime. I have added in druntime/src/core/thread.d to

final Thread start() nothrow

of class Thread

         import core.stdc.stdio;
         printf("start Thread\n");


And to

~this() nothrow @nogc

         import core.stdc.stdio;
         printf("detach Thread\n");

I recompiled phobos to apply changes to druntime and I see that 
"start Thread" there is, but "detach Thread" printed only at the 
end of process even if I do GC.collect().

So dtor of class Thread doesn't call on GC. Why?