This reverts commit 228d13f6ed914d1e7f6bd2416e3f5be8283be865.

This commit makes default.c and mmtk.c depend on shape.h, which prevents
them from building independently.

Attempt to fix the following SEGV:

```
ruby(gc_mark) ../src/gc/default/default.c:4429
ruby(gc_mark_children+0x45) [0x560b380bf8b5] ../src/gc/default/default.c:4625
ruby(gc_mark_stacked_objects) ../src/gc/default/default.c:4647
ruby(gc_mark_stacked_objects_all) ../src/gc/default/default.c:4685
ruby(gc_marks_rest) ../src/gc/default/default.c:5707
ruby(gc_marks+0x4e7) [0x560b380c41c1] ../src/gc/default/default.c:5821
ruby(gc_start) ../src/gc/default/default.c:6502
ruby(heap_prepare+0xa4) [0x560b380c4efc] ../src/gc/default/default.c:2074
ruby(heap_next_free_page) ../src/gc/default/default.c:2289
ruby(newobj_cache_miss) ../src/gc/default/default.c:2396
ruby(RB_SPECIAL_CONST_P+0x0) [0x560b380c5df4] ../src/gc/default/default.c:2420
ruby(RB_BUILTIN_TYPE) ../src/include/ruby/internal/value_type.h:184
ruby(newobj_init) ../src/gc/default/default.c:2136
ruby(rb_gc_impl_new_obj) ../src/gc/default/default.c:2500
ruby(newobj_of) ../src/gc.c:996
ruby(rb_imemo_new+0x37) [0x560b380d8bed] ../src/imemo.c:46
ruby(imemo_fields_new) ../src/imemo.c:105
ruby(rb_imemo_fields_new) ../src/imemo.c:120
```

I have no reproduction, but my understanding based on the backtrace
and error is that GC is triggered inside `newobj_init` causing the
new object to be marked while in a incomplete state.

I believe the fix is to pass the `shape_id` down to `newobj_init`
so it can be set before the GC has a chance to trigger.

The "EXIVAR" terminology has been replaced by "gen fields"
AKA "generic fields".

Exivar implies variable, but generic fields include more than
just variables, e.g. `object_id`.

[Bug #21710]

- struct.c: `struct_alloc`

It is possible for a `NEWOBJ` tracepoint call back to write fields
into a newly allocated object before `struct_alloc` had the time
to set the `RSTRUCT_GEN_FIELDS` flags and such.

Hence we can't blindly initialize the `fields_obj` reference to `0`
we first need to check no fields were added yet.

- object.c: `rb_class_allocate_instance`

Similarly, if a `NEWOBJ` tracepoint tries to set fields on the object,
the `shape_id` must already be set, as it's required on T_OBJECT to
know where to write fields.

`NEWOBJ_OF` had to be refactored to accept a `shape_id`.

We know the argument is not a class, module or special const, so we can
skip these checks.

The type name bindgen picks for anonymous enums creates desync issues on
the bindgen CI checks.

While accessing the ivars of other types is too complicated to
realistically generate the ASM for it, we can at least provide
the ivar index as to not have to lookup the shape tree every
time.

```
compare-ruby: ruby 3.5.0dev (2025-08-27T14:58:58Z merge-vm-setivar-d.. 5b749d8e53) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-28T17:58:32Z yjit-get-exivar efaa8c9b09) +YJIT +PRISM [arm64-darwin24]

|                           |compare-ruby|built-ruby|
|:--------------------------|-----------:|---------:|
|vm_ivar_get_on_obj         |     930.458|   936.865|
|                           |           -|     1.01x|
|vm_ivar_get_on_class       |     134.471|   431.622|
|                           |           -|     3.21x|
|vm_ivar_get_on_generic     |     146.679|   284.408|
|                           |           -|     1.94x|
```

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>