build: compile V8 using system compiler

Stop using the copy of clang that is bundled with Google's build tools
and start using the compiler that is installed on the buildbots.
It stopped working on one of the machines because it looked in the
wrong place for system headers and is not representative of how users
build Node.js on their systems.

PR-URL: https://github.com/nodejs/node/pull/17489


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>