search_nonascii(): Replace UB pointer cast with memcpy

	Casting a pointer to create an unaligned one is undefined behavior in C
	standards. Use memcpy to express the unaligned load instead to play by
	the rules.

	Practically, this yields the same binary output in many situations
	while fixing the crash in [Bug #21715].

	Fix C23 (GCC 15) WIN32 compatibility for rb_define_* functions

	Fixes [Bug #21286]

	[Bug #19426] Fix endless `Range#step` with `#succ` method

	[Bug #20025] Check if upper/lower before fallback to case-folding

	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>

	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`