macOS’s `readlink` doesn’t support the `-f` flag. By using `realpath`
we get the same functionality with added support for macOS.

Finally, this patch adds `v8` and `v8-debug` to the completion
command. These are the binary names used by jsvu [1].

[1]: https://github.com/GoogleChromeLabs/jsvu

Bug: v8:11567
Change-Id: I1ec47752e50bce4bfb97a7ce23571d0a6e55dbc1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2762147
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73438}