cmd/go: skip external tests on freebsd-arm builder

It is just far too slow.
I have a CL for Go 1.6 that makes many of these into internal tests.
That will improve the coverage.

It does not matter much, because basically none of the go command
tests are architecture dependent, so the other builders will catch
any problems.

Fixes freebsd-arm builder.

Change-Id: I8b2f6ac2cc1e7657019f7731c6662dc43e20bfb5
Reviewed-on: https://go-review.googlesource.com/13166Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -24,17 +24,16 @@ import (
 	"time"
 )
 
-// Whether we can run go or ./testgo.
-var canRun = true
+var (
+	canRun  = true  // whether we can run go or ./testgo
+	canRace = false // whether we can run the race detector
+	canCgo  = false // whether we can use cgo
 
-// The suffix for executables, because Windows.
-var exeSuffix string
+	exeSuffix string // ".exe" on Windows
 
-// Whether we can run the race detector.
-var canRace bool
-
-// Whether we can use cgo.
-var canCgo bool
+	builder             = testenv.Builder()
+	skipExternalBuilder = false // skip external tests on this builder
+)
 
 func init() {
 	switch runtime.GOOS {
@@ -47,6 +46,11 @@ func init() {
 		}
 	}
 
+	if strings.HasPrefix(builder+"-", "freebsd-arm-") {
+		skipExternalBuilder = true
+		canRun = false
+	}
+
 	switch runtime.GOOS {
 	case "windows":
 		exeSuffix = ".exe"
@@ -134,6 +138,10 @@ type testgoData struct {
 func testgo(t *testing.T) *testgoData {
 	testenv.MustHaveGoBuild(t)
 
+	if skipExternalBuilder {
+		t.Skip("skipping external tests on %s builder", builder)
+	}
+
 	return &testgoData{t: t}
 }