cmd/releasebot, cmd/release: add darwin-arm64 target for Go 1.16

For golang/go#42756.

Change-Id: Ia28fb6878617715fb674ccfad0c5801c8925270b
Reviewed-on: https://go-review.googlesource.com/c/build/+/278436
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cmd/release/release.go b/cmd/release/release.go
index 59bb402..7680406 100644
--- a/cmd/release/release.go
+++ b/cmd/release/release.go
@@ -207,6 +207,13 @@
 		Builder: "darwin-amd64-10_15",
 	},
 	{
+		GoQuery: ">= go1.16beta1", // Go 1.16 Beta 1 is the first Go (pre-)release with the darwin/arm64 port.
+		OS:      "darwin",
+		Arch:    "arm64",
+		Race:    true,
+		Builder: "darwin-arm64-11_0-toothrot",
+	},
+	{
 		OS:        "linux",
 		Arch:      "s390x",
 		SkipTests: true,