Push a real iseq in rb_vm_push_frame_fname()

	Previously, vm_make_env_each() (used during proc
	creation and for the debug inspector C API) picked up the
	non-GC-allocated iseq that rb_vm_push_frame_fname() creates,
	which led to a SEGV when the GC tried to mark the non GC object.

	Put a real iseq imemo instead. Speed should be about the same since
	the old code also did a imemo allocation and a malloc allocation.

	Real iseq allows ironing out the special-casing of dummy frames in
	rb_execution_context_mark() and rb_execution_context_update(). A check
	is added to RubyVM::ISeq#eval, though, to stop attempts to run dummy
	iseqs.

	[Bug #21180]

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>