This way d8 can use a read-only mapping for its input file. Otherwise,
it would require write access for reading the sources.

Bug: v8:8997
Change-Id: I8e117f0a3d9c125ffd1dc1682d5af04b3f6287c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1530804
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60331}