This is a reland of 9fe53c4f

- Fix data-race by using an atomic for flag_hash;
- Make sure flag_hash != 0
- Initialize flag_hash in V8::InitializeOncePerProcessImpl
- Clear flag_hash in more cases

Original change's description:
> [flags] Skip --random-seed in FlagList::Hash
>
> Node and friends use --random-seed to temporary reset the seed for
> predictable code-cache creation. To allow custom random seeds at runtime
> the flag is reset for encoding the FlagList::Hash in the snapshots.
>
> We will soon disallow changing flags via the API after V8 has been
> initialized. In order to make node work we will exclude --random-seed
> from the FlagList::Hash calculation.
>
> Drive-by-fix:
> * Lazily initialize flag_hash instead of calculating it after every call
>   to SetFlagsFromString / EnforceFlagImplications.
> * Simplify hash string source creation since out << flag now includes
>   the full flag information
>
> Bug: v8:12309
> Change-Id: I1a168f4702d8c4d160ff12fdbea881731e4ea8b6
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218159
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#77345}

Bug: v8:12309
Change-Id: I12cd2931d81dc74e07a4da3564e4bf8dd151300a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218981
Commit-Queue: Marja Hölttä <marja@chromium.org>
Auto-Submit: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77373}