Fix RUBY_FREE_AT_EXIT with ASAN

When Ruby is built with ASAN and RUBY_FREE_AT_EXIT is enabled, the
following error occurs:

    READ of size 8 at 0x74c666610020 thread T0
        #0 0x593b6712ecc6 in RB_BUILTIN_TYPE include/ruby/internal/value_type.h:191:30
        #1 0x593b6712ecc6 in rb_gc_impl_shutdown_free_objects gc_impl.c:3208:17
        #2 0x593b6749a62e in ruby_vm_destruct vm.c:3133:17

gc_impl.c

@@ -3205,8 +3205,10 @@ rb_gc_impl_shutdown_free_objects(void *objspace_ptr)
         uintptr_t pend = p + page->total_slots * stride;
         for (; p < pend; p += stride) {
             VALUE vp = (VALUE)p;
-            if (RB_BUILTIN_TYPE(vp) != T_NONE) {
-                rb_gc_obj_free(objspace, vp);
+            asan_unpoisoning_object(vp) {
+                if (RB_BUILTIN_TYPE(vp) != T_NONE) {
+                    rb_gc_obj_free(objspace, vp);
+                }
             }
         }
     }