Seal read-only space before startup deserialization. This is necessary
for fully isolating read-only space creation. Strings within read-only
space are now eagerly rehashed.

Bug: v8:7464
Change-Id: I8b91a6f6b31e03e69d80109b1ca30c675a495c36
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1511485Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#60182}