The default protobuf compiler main builds generators for the CLI tool
for all the supported languages.

This CL adds a custom main() which only requires the CPP generator.
The protobuf compiler is a build-time dependency so this won't shrink
binary size but will speed up compile a little bit and means we don't
have to list all of the files for the other supported languages in the
build.gn file. That should hopefully make protobuf rolls a bit easier
given that we are maintaining GN build files for protobuf.

Cq-Include-Trybots: luci.v8.try:v8_linux64_perfetto_dbg_ng
Bug: v8:8339
Change-Id: Iac432dfe53b7ac9cfa819be1b2945d5948ffff38
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1599448
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61317}