When a forked process was started in a thread, this would result in a
double-free during the child process exit.

    RUBY_FREE_AT_EXIT=1 ./miniruby -e 'Thread.new { fork { } }.join; Process.waitpid'

This is because the main thread in the forked process was not the
initial VM thread, and the new thread's stack was freed as part of
objectspace iteration.

This change also allows rb_threadptr_root_fiber_release to run without
EC being available.