Piggybacking of splitting heap setup, this change adds a shared
read-only heap and a flag to enable it.

Also makes CallOnce use std::function instead of a raw function
pointer so the CL can use lambdas with CallOnce.

Bug: v8:7464
Change-Id: I9a97fb1baa6badca39a7381de3fd9e01f5969340
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1518180Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#60241}