class.c: rb_class_duplicate_classext also dup content of cvc_tbl

	[Bug #21952]

	Shallow copying the table result in the same memory being shared
	between multiple box, causing double free when one of the box
	is garbage collected.

	---

	class.c: Make cvc_tbl a managed object

	[Bug #21952]

	Solves the double-free or use after-free concern with boxes.
	Now entries can safely be used for copy-on-write.

	Also is likely necessary to make it save to read cvar from
	secondary ractors, as allowed since: ab32c0e690b805cdaaf264ad4c3421696c588204

jump, branchif, etc don't invalidate locals in the JIT; they might in the interpreter because they can execute arbitrary code, but the JIT side exits before that happens.

Co-authored-by: Alan Wu <alanwu@ruby-lang.org>

To fix warnings from rustc on e.g. Make 4.3, which is in Ubuntu 24.04:

> warning: failed to connect to jobserver from environment variable

* To avoid calling rb_gc_writebarrier() with an immediate value in gen_write_barrier(),
  and avoid the LIR jump issue.

* This reverts commit 623559faa3dd0927b4034a752226a30ae8821604.
* There is an issue with the jump in LIR, see https://github.com/ruby/ruby/pull/15542.

This reverts commit 2f151e76b5dc578026706b31f054d5caf5374b05.

The SP decrement (push) before the call do not match up with
the pops after the call, so registers were restored incorrectly.

Code from:

    ./miniruby --zjit-call-threshold=1 --zjit-dump-disasm -e 'p Time.new(1992, 9, 23, 23, 0, 0, :std)'

    str x11, [sp, #-0x10]!
    str x12, [sp, #-0x10]!
    stur x7, [sp]              # last argument
    mov x0, x20
    mov x7, x6
    mov x6, x5
    mov x5, x4
    mov x4, x3
    mov x3, x2
    mov x2, x1
    ldur x1, [x29, #-0x20]
    mov x16, #0xccfc
    movk x16, #0x2e7, lsl #16
    movk x16, #1, lsl #32
    blr x16
    ldr x12, [sp], #0x10       # supposed to match str x12, [sp, #-0x10]!, but got last argument
    ldr x11, [sp], #0x10