Add missing lock in `rb_gc_impl_define_finalizer`

`objspace->finalizer_table` must be synchronized,
otherwise concurrent insertion from multiple ractors
will cause a crash.

Repro:

```ruby
ractors = 5.times.map do |i|
  Ractor.new do
    100_000.times.map do
      o = Object.new
      ObjectSpace.define_finalizer(o, ->(id) {})
      o
    end
  end
end

ractors.each(&:take)
```

1 files changed, 4 insertions, 0 deletions

diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index b844c8c2a3..70df47563d 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -973,6 +973,8 @@ rb_gc_impl_define_finalizer(void *objspace_ptr, VALUE obj, VALUE block)
 
     RBASIC(obj)->flags |= FL_FINALIZE;
 
+    int lev = rb_gc_vm_lock();
+
     if (st_lookup(objspace->finalizer_table, obj, &data)) {
         table = (VALUE)data;
 
@@ -997,6 +999,8 @@ rb_gc_impl_define_finalizer(void *objspace_ptr, VALUE obj, VALUE block)
         st_add_direct(objspace->finalizer_table, obj, table);
     }
 
+    rb_gc_vm_unlock(lev);
+
     return block;
 }