dashboard: create misc-compile-darwinarm64 builder
dashboard: create misc-compile-darwinarm64 builder

Create the builder as a post-submit builder with a non-zero KnownIssue
value first. Once we know it works well, we'll have the confidence to
be able to safely promote it to a normal TryBot, without the risk of
causing disruption for contributors.

It's different from misc-compile-darwin since it needs to run on recent
Go 1.16 commits only.

For golang/go#42341.

Change-Id: I67170988d7323cb73439755a3961532c668040e7
Reviewed-on: https://go-review.googlesource.com/c/build/+/267105
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build