cmd/go: add darwin/arm64 in the list of supported systems in help message
Fixes #49173.
Change-Id: I71270b4ff7e9ede3cdfa5946b73142a731752adf
Reviewed-on: https://go-review.googlesource.com/c/go/+/358901 Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com> Reviewed-by: Keith Randall <khr@golang.org>
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>