dashboard: add compile-only TryBot coverage for windows/arm64 port

The windows/arm64 port has been completed in golang.org/issue/36439
recently. Add pre-submit compile-only coverage via a misc-compile
TryBot for the new port. It can be upgraded to a normal TryBot that
also executes tests after a windows/arm64 builder becomes available.

Tested locally by running 'buildall.bash windows-arm64' on a recent
Go tip commit (CL 313849), which passed.

For golang/go#36439.
Updates golang/go#42604.

Change-Id: Ie4988a876aa18392a583eb407c6c3621eb30a4f8
Reviewed-on: https://go-review.googlesource.com/c/build/+/314430
Trust: Dmitri Shuralyov <[email protected]>
Trust: Alexander Rakoczy <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Alexander Rakoczy <[email protected]>

Author: dmitshur

dashboard/builders.go

@@ -1651,7 +1651,7 @@ func init() {
 	// some misc-compile TryBot could become much slower than others.)
 	//
 	// See golang.org/issue/32632.
-	addMiscCompile("-mac-win", "darwin-amd64", "windows-arm")
+	addMiscCompile("-mac-win", "darwin-amd64", "windows-arm", "windows-arm64")
 	addMiscCompileGo1(16, "-darwinarm64", "darwin-arm64") // darwin/arm64 (for Go 1.16 and newer) only.
 	addMiscCompile("-mips", "linux-mips", "linux-mips64")
 	addMiscCompile("-mipsle", "linux-mipsle", "linux-mips64le")

dashboard/builders_test.go

@@ -843,12 +843,10 @@ func TestTryBotsCompileAllPorts(t *testing.T) {
 	// knownMissing tracks Go ports that that are known to be
 	// completely missing TryBot (pre-submit) test coverage.
 	//
-	// All ports should have either a real TryBot or at least a misc-compile TryBot,
+	// All completed ports should have either a real TryBot or at least a misc-compile TryBot,
 	// so this map is meant to be used to temporarily fix tests
 	// when the work of adding a new port is actively underway.
-	knownMissing := map[string]bool{
-		"windows-arm64": true, // TODO(golang.org/issue/42604): Add builder for windows/arm64.
-	}
+	knownMissing := map[string]bool{}
 
 	var done = make(map[string]bool)
 	check := func(goos, goarch string) {