fix GC event synchronization

(1) gc_verify_internal_consistency() use barrier locking
    for consistency while `during_gc == true` at the end
    of the sweep on `RGENGC_CHECK_MODE >= 2`.
(2) `rb_objspace_reachable_objects_from()` is called without
    VM synchronization and it checks `during_gc != true`.

So (1) and (2) causes BUG because of `during_gc == true`.
To prevent this error, wait for VM barrier on `during_gc == false`
and introduce VM locking on `rb_objspace_reachable_objects_from()`.

http://ci.rvm.jp/results/trunk-asserts@phosphorus-docker/3830088

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