This is a reland of e5d568f4

Original change's description:
> heap: Implement DiscardSystemPages for Fuchsia
>
> Bug: v8:12050
> Change-Id: I9fa5c0d38f277e363befbdaecaaedd4ee55bfb07
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3071208
> Reviewed-by: Wez <wez@chromium.org>
> Commit-Queue: Omer Katz <omerkatz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#76160}

Bug: v8:12050
Change-Id: I8ca9a578bc36e9f85634dfa3fba0baef6bb82579
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3081521Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76170}