cmd/dist: add support for openbsd/arm64

Updates #31656

Change-Id: If481df050cd879f7c7c22a79c17c33af00a8b389
Reviewed-on: https://go-review.googlesource.com/c/go/+/174125
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/build.go

@@ -1514,6 +1514,7 @@ var cgoEnabled = map[string]bool{
 	"openbsd/386":     true,
 	"openbsd/amd64":   true,
 	"openbsd/arm":     true,
+	"openbsd/arm64":   false,
 	"plan9/386":       false,
 	"plan9/amd64":     false,
 	"plan9/arm":       false,

src/cmd/dist/main.go

@@ -71,8 +71,9 @@ func main() {
 	case "openbsd":
 		// The gcc available on OpenBSD armv7 is old/inadequate (for example, lacks
 		// __sync_fetch_and_*/__sync_*_and_fetch) and will likely be removed in the
-		// not-to-distant future - use clang instead.
-		if runtime.GOARCH == "arm" {
+		// not-to-distant future - use clang instead. OpenBSD arm64 does not ship
+		// with gcc.
+		if runtime.GOARCH == "arm" || runtime.GOARCH == "arm64" {
 			defaultclang = true
 		}
 	case "plan9":