[Feature #19783]

This commit adds support for weak references in the GC through the
function `rb_gc_mark_weak`. Unlike strong references, weak references
does not mark the object, but rather lets the GC know that an object
refers to another one. If the child object is freed, the pointer from
the parent object is overwritten with `Qundef`.

Co-Authored-By: Jean Boussier <byroot@ruby-lang.org>

These functions manipulate darray without the possibility of triggering
GC, which is used for places that cannot trigger GC. These functions
crash when the allocation fails.

darray was used in YJIT which required the functions to not trigger GC.
YJIT has now moved to Rust and does not use darray anymore, so we can
remove the functions that don't trigger GC and only keep the ones that
trigger GC.

Using a fake (malloc) RArray is not friendly for the garbage
collector. Fake RArray does not have a heap page, so it causes Variable
Width Allocation to crash when we try to implement it on Arrays.

This commit changes feature_index from a RArray to a darray.

Changes size and capacity of darray to size_t to support more
elements.

Adds functions to darray that use GC allocation functions.

* Malloc branch entries

* Add ASM comment for stack overflow check

* WIP

* Fix branch GC code. Add rb_darray_remove_unordered().

* Fix block end_pos after branch rewriting. Remove dst_patched bits.