dashboard: add a make.bash-only builder on real ARM hardware

This is a new builder in prep for the change to the "linux-arm"
builder where the GOARCH=arm make.bash will be cross-compiled from a
Kubernetes container on fast hardware.

Updates golang/go#17105 (cross-compile ARM builders' make.bash)
Updates golang/go#17104 (5 minute trybots)

Change-Id: Icfd2644d77639f731151abe54839322960418254
Reviewed-on: https://go-review.googlesource.com/29670
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

cmd/coordinator/coordinator.go

@@ -1538,6 +1538,9 @@ func (st *buildStatus) runAllSharded() (remoteErr, err error) {
 	if remoteErr != nil {
 		return fmt.Errorf("build failed: %v", remoteErr), nil
 	}
+	if st.conf.StopAfterMake {
+		return nil, nil
+	}
 
 	if err := st.doSnapshot(); err != nil {
 		return nil, err

dashboard/builders.go

@@ -275,6 +275,14 @@ type BuildConfig struct {
 	CompileOnly bool // if true, compile tests, but don't run them
 	FlakyNet    bool // network tests are flaky (try anyway, but ignore some failures)
 
+	// StopAfterMake causes the build to stop after the make
+	// script completes, returning its result as the result of the
+	// whole build. It does not run or compile any of the tests,
+	// nor does it write a snapshot of the world to cloud
+	// storage. This option is only supported for builders whose
+	// BuildConfig.SplitMakeRun returns true.
+	StopAfterMake bool
+
 	// numTestHelpers is the number of _additional_ buildlets
 	// past the first one to help out with sharded tests.
 	// For trybots, the numTryHelpers value is used, unless it's
@@ -628,6 +636,12 @@ func init() {
 		numTestHelpers:    2,
 		numTryTestHelpers: 7,
 	})
+	addBuilder(BuildConfig{
+		Name:          "linux-arm-nativemake",
+		Notes:         "runs make.bash on real ARM hardware, but does not run tests",
+		HostType:      "host-linux-arm",
+		StopAfterMake: true,
+	})
 	addBuilder(BuildConfig{
 		Name:     "linux-arm-arm5",
 		HostType: "host-linux-arm",