We now have a src/d8 directory, and having "d8" in .gitignore makes git
ignore all new files in this directory, and also hides the directory in
some IDEs.

R=yangguo@chromium.org

No-Try: true
Change-Id: I3106378417c1fe9ebc369275eb52d5438cee8825
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627530Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61786}