merge revision(s) bccec7fb, 5f8ebcad, e13575bb, 4adcfc8c: [Backport #19584]

	Fix crash in rb_gc_register_address

	[Bug #19584]

	Some C extensions pass a pointer to a global variable to
	rb_gc_register_address. However, if a GC is triggered inside of
	rb_gc_register_address, then the object could get swept since it does
	not exist on the stack.

	[Bug #19584] Register global variable address before assignment

	[Bug #19584] Register global variables before assignment

	[Bug #19584] [DOC] Tweek description of `rb_gc_register_address`