Add linux-amd64-racecompile

This builder is very special: it installs (compiles) the compiler and linker packages with -race, then rebuilds Go again using those race-detecting compiler bits. (Instead of running any ordinary tests.) This is done to test concurrent compilation for races.

https://github.com/golang/build/blob/83a8520724285855120f774cc4a7b57540a1d50b/dashboard/builders.go

		Name:                "linux-amd64-racecompile",
		HostType:            "host-linux-jessie",
		tryBot:              nil, // TODO: add a func to conditionally run this trybot if compiler dirs are touched
		CompileOnly:         true,
		SkipSnapshot:        true,
		StopAfterMake:       true,
		InstallRacePackages: []string{"cmd/compile", "cmd/link"}, ...

func (c *BuildConfig) GoInstallRacePackages() []string {
	if c.InstallRacePackages != nil {
		return append([]string(nil), c.InstallRacePackages...)
	}
	if c.IsRace() {
		return []string{"std"}
	}
	return nil
}

https://github.com/golang/build/blob/83a8520724285855120f774cc4a7b57540a1d50b/internal/buildgo/buildgo.go

func (gb GoBuilder) RunMake(ctx context.Context, bc *buildlet.Client, w io.Writer) (remoteErr, err error) {
...
	// Need to run "go install -race std" before the snapshot + tests.
	if pkgs := gb.Conf.GoInstallRacePackages(); len(pkgs) > 0 {
		sp := gb.CreateSpan("install_race_std")
		remoteErr, err = bc.Exec(ctx, path.Join(gb.Goroot, "bin/go"), buildlet.ExecOpts{
			Output:   w,
			ExtraEnv: append(gb.Conf.Env(), "GOBIN="),
			Debug:    true,
			Args:     append([]string{"install", "-race"}, pkgs...),
		})
...
	if gb.Name == "linux-amd64-racecompile" {
		return gb.runConcurrentGoBuildStdCmd(ctx, bc, w)
	}

// runConcurrentGoBuildStdCmd is a step specific only to the
// "linux-amd64-racecompile" builder to exercise the Go 1.9's new
// concurrent compilation. It re-builds the standard library and tools
// with -gcflags=-c=8 using a race-enabled cmd/compile and cmd/link
// (built by caller, RunMake, per builder config).
// The idea is that this might find data races in cmd/compile and cmd/link.
func (gb GoBuilder) runConcurrentGoBuildStdCmd(ctx context.Context, bc *buildlet.Client, w io.Writer) (remoteErr, err error) {

Start with dashboard/builders.go and internal/buildgo/buildgo.go, especially GoInstallRacePackages, RunMake, and runConcurrentGoBuildStdCmd. Compare the existing builder configuration and execution flow, then verify that linux-amd64-racecompile installs the race-enabled compiler and linker and rebuilds the standard library with concurrent compilation.