compile.c: use putspecialobject for RubyVM::FrozenCore

[Bug #20569]

`putobject RubyVM::FrozenCore`, is not serializable, we
have to use `putspecialobject VM_SPECIAL_OBJECT_VMCORE`.

compile.c

@@ -9750,7 +9750,12 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const no
  case NODE_LIT:{
  debugp_param("lit", node->nd_lit);
  if (!popped) {
- ADD_INSN1(ret, node, putobject, node->nd_lit);
+ if (UNLIKELY(node->nd_lit == rb_mRubyVMFrozenCore)) {
+ ADD_INSN1(ret, node, putspecialobject, INT2FIX(VM_SPECIAL_OBJECT_VMCORE)); // [Bug #20569]
+ }
+ else {
+ ADD_INSN1(ret, node, putobject, node->nd_lit);
+ }
  RB_OBJ_WRITTEN(iseq, Qundef, node->nd_lit);
  }
  break;

test/ruby/test_iseq.rb

@@ -167,6 +167,14 @@ def obj.foo(*) nil.instance_eval{ ->{super} } end
  end
  end
 
+ def test_ractor_shareable_value_frozen_core
+ iseq = RubyVM::InstructionSequence.compile(<<~'RUBY')
+ # shareable_constant_value: literal
+ REGEX = /#{}/ # [Bug #20569]
+ RUBY
+ assert_includes iseq.to_binary, "REGEX".b
+ end
+
  def test_disasm_encoding
  src = "\u{3042} = 1; \u{3042}; \u{3043}"
  asm = compile(src).disasm