This solves multiple problems.

First, RB_VM_LOCK_ENTER/LEAVE is a barrier. We could at least use the
_NO_BARRIER variant.

Second, this doesn't need to interfere with GC or other GVL users when
multiple Ractors are used. This needs to be used in very few places, so
the benefit of fine-grained locking would outweigh its small maintenance
cost.

Third, it fixes a crash for YJIT. Because YJIT is never disabled until a
process exits unlike MJIT that finishes earlier, we could call jit_cont_free
when EC no longer exists, which crashes RB_VM_LOCK_ENTER.