Add CHECK ensuring that the young generation is indeed empty for
pointers updating. This is necessary as otherwise iterating an
object may race with updating a slot in a Map for WasmStruct.

Bug: v8:12185
Change-Id: Id590cf267fedf95d97df2464a638352696ad53db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3260514Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77709}