YJIT: Move RefCell one level down

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`).

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