Enable handle fork by GC

ECL is multithread application which uses fork() to run subprocesses for example. fork() creates a copy of an application with only one thread, but GC may have pointers inside to old threads which may lead to corrupted memory and other quite bad things.

Thus, on Darwin GC is used mach_thread_self() to get thread id, and returned value migth be different after fork() for the same thread.

Closes: #718 (closed)