Previously, the d8 prompt was printed without flushing stdout. This
relies on the platform's libc to flush stdout when reading from stdin.
This behavior is not portable and breaks the prompt on some platforms.

Change-Id: Ieddf7ec5a6eab15796e69742bb4c9546ceb54c37
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3752006Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81634}