Add COPYRIGHT file to the root directory; Update toolchain;

Replaces the former toolchain information with the current one.

This copyright is currently available on
https://www.freebsd.org/copyright/freebsd-doc-license/ or
website/content/en/copyright/freebsd-doc-license.adoc.

Reviewed by: carlavilla
Discussed by: doceng
Differential Revision: https://reviews.freebsd.org/D32752