On windows, the range to be discarded needs to be split by the
reservations, analogous to committing. This CL reuses the same logic,
and reenables discarding pages on all platforms.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: I11716d6381f765bdfe4cf48502b5cdc1f42cf8ab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1611682
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61526}