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This is the second part of making YJIT work with parallel GC. During GC, `rb_yjit_iseq_mark` and `rb_yjit_iseq_update_references` need to resolve offsets in `Block::gc_obj_offsets` into absolute addresses before reading or updating the fields. This needs the base address stored in `VirtualMemory::region_start` which was previously behind a `RefCell`. When multiple GC threads scan multiple iseq simultaneously (which is possible for some GC modules such as MMTk), it will panic because the `RefCell` is already borrowed. We notice that some fields of `VirtualMemory`, such as `region_start`, are never modified once `VirtualMemory` is constructed. We change the type of the field `CodeBlock::mem_block` from `Rc<RefCell<T>>` to `Rc<T>`, and push the `RefCell` into `VirtualMemory`. We extract mutable fields of `VirtualMemory` into a dedicated struct `VirtualMemoryMut`, and store them in a field `VirtualMemory::mutable` which is a `RefCell<VirtualMemoryMut>`. After this change, methods that access immutable fields in `VirtualMemory`, particularly `base_ptr()` which reads `region_start`, will no longer need to borrow any `RefCell`. Methods that access mutable fields will need to borrow `VirtualMemory::mutable`, but the number of borrowing operations becomes strictly fewer than before because borrowing operations previously done in callers (such as `CodeBlock::write_mem`) are moved into methods of `VirtualMemory` (such as `VirtualMemory::write_bytes`).