test: add -target flag.

--- test/run.go
+++ test/run.go
@@ -37,9 +37,9 @@ var (
 	numParallel    = flag.Int("n", runtime.NumCPU(), "number of parallel tests to run")
 	summary        = flag.Bool("summary", false, "show summary of results")
 	showSkips      = flag.Bool("show_skips", false, "show skipped tests")
-	linkshared     = flag.Bool("linkshared", false, "")
 	updateErrors   = flag.Bool("update_errors", false, "update error messages in test file based on compiler output")
 	runoutputLimit = flag.Int("l", defaultRunOutputLimit(), "number of parallel runoutput tests to run")
+	target         = flag.String("target", "", "if non empty, use 'go_target' to compile test files and 'go_target_exec' to run the binaries")
 
 	shard  = flag.Int("shard", 0, "shard index to run. Only applicable if -shards is non-zero.")
 	shards = flag.Int("shards", 0, "number of shards. If 0, all tests are run. This is used by the continuous build.")
@@ -192,19 +192,11 @@ func goFiles(dir string) []string {
 type runCmd func(...string) ([]byte, error)
 
 func compileFile(runcmd runCmd, longname string) (out []byte, err error) {
-	cmd := []string{"go", "tool", "compile", "-e"}
-	if *linkshared {
-		cmd = append(cmd, "-dynlink", "-installsuffix=dynlink")
-	}
-	cmd = append(cmd, longname)
-	return runcmd(cmd...)
+	return runcmd(findGoCmd(), "tool", "compile", "-e", longname)
 }
 
 func compileInDir(runcmd runCmd, dir string, names ...string) (out []byte, err error) {
-	cmd := []string{"go", "tool", "compile", "-e", "-D", ".", "-I", "."}
-	if *linkshared {
-		cmd = append(cmd, "-dynlink", "-installsuffix=dynlink")
-	}
+	cmd := []string{findGoCmd(), "tool", "compile", "-e", "-D", ".", "-I", "."}
 	for _, name := range names {
 		cmd = append(cmd, filepath.Join(dir, name))
 	}
@@ -213,15 +205,21 @@ func compileInDir(runcmd runCmd, dir string, names ...string) (out []byte, err e
 
 func linkFile(runcmd runCmd, goname string) (err error) {
 	pfile := strings.Replace(goname, ".go", ".o", -1)
-	cmd := []string{"go", "tool", "link", "-w", "-o", "a.exe", "-L", "."}
-	if *linkshared {
-		cmd = append(cmd, "-linkshared", "-installsuffix=dynlink")
-	}
-	cmd = append(cmd, pfile)
-	_, err = runcmd(cmd...)
+	_, err = runcmd(findGoCmd(), "tool", "link", "-w", "-o", "a.exe", "-L", ".", pfile)
 	return
 }
 
+func goRun(runcmd runCmd, goname string, args ...string) (out []byte, err error) {
+	cmd := []string{findGoCmd(), "run"}
+	if len(findExecCmd()) > 0 {
+		cmd = append(cmd, "-exec")
+		cmd = append(cmd, findExecCmd()...)
+	}
+	cmd = append(cmd, goname)
+	cmd = append(cmd, args...)
+	return runcmd(cmd...)
+}
+
 // skipError describes why a test was skipped.
 type skipError string
 
@@ -530,8 +528,7 @@ func (t *test) run() {
 		t.err = fmt.Errorf("unimplemented action %q", action)
 
 	case "errorcheck":
-		cmdline := []string{"go", "tool", "compile", "-e", "-o", "a.o"}
-		// No need to add -dynlink even if linkshared if we're just checking for errors...
+		cmdline := []string{findGoCmd(), "tool", "compile", "-e", "-o", "a.o"}
 		cmdline = append(cmdline, flags...)
 		cmdline = append(cmdline, long)
 		out, err := runcmd(cmdline...)
@@ -640,19 +637,14 @@ func (t *test) run() {
 		}
 
 	case "build":
-		_, err := runcmd("go", "build", "-o", "a.exe", long)
+		_, err := runcmd(findGoCmd(), "build", "-o", "a.exe", long)
 		if err != nil {
 			t.err = err
 		}
 
 	case "run":
 		useTmp = false
-		cmd := []string{"go", "run"}
-		if *linkshared {
-			cmd = append(cmd, "-linkshared")
-		}
-		cmd = append(cmd, t.goFileName())
-		out, err := runcmd(append(cmd, args...)...)
+		out, err := goRun(runcmd, t.goFileName(), args...)
 		if err != nil {
 			t.err = err
 			return
@@ -667,12 +659,7 @@ func (t *test) run() {
 			<-rungatec
 		}()
 		useTmp = false
-		cmd := []string{"go", "run"}
-		if *linkshared {
-			cmd = append(cmd, "-linkshared")
-		}
-		cmd = append(cmd, t.goFileName())
-		out, err := runcmd(append(cmd, args...)...)
+		out, err := goRun(runcmd, t.goFileName(), args...)
 		if err != nil {
 			t.err = err
 			return
@@ -682,12 +669,7 @@ func (t *test) run() {
 			t.err = fmt.Errorf("write tempfile:%s", err)
 			return
 		}
-		cmd = []string{"go", "run"}
-		if *linkshared {
-			cmd = append(cmd, "-linkshared")
-		}
-		cmd = append(cmd, tfile)
-		out, err = runcmd(cmd...)
+		out, err = goRun(runcmd, tfile)
 		if err != nil {
 			t.err = err
 			return
@@ -698,12 +680,7 @@ func (t *test) run() {
 
 	case "errorcheckoutput":
 		useTmp = false
-		cmd := []string{"go", "run"}
-		if *linkshared {
-			cmd = append(cmd, "-linkshared")
-		}
-		cmd = append(cmd, t.goFileName())
-		out, err := runcmd(append(cmd, args...)...)
+		out, err := goRun(runcmd, t.goFileName(), args...)
 		if err != nil {
 			t.err = err
 			return
@@ -714,7 +691,7 @@ func (t *test) run() {
 			t.err = fmt.Errorf("write tempfile:%s", err)
 			return
 		}
-		cmdline := []string{"go", "tool", "compile", "-e", "-o", "a.o"}
+		cmdline := []string{findGoCmd(), "tool", "compile", "-e", "-o", "a.o"}
 		cmdline = append(cmdline, flags...)
 		cmdline = append(cmdline, tfile)
 		out, err = runcmd(cmdline...)
@@ -741,6 +718,10 @@ func findExecCmd() []string {
 		return execCmd
 	}
 	execCmd = []string{} // avoid work the second time
+	if *target != "" {
+		execCmd = []string{"go_" + *target + "_exec"}
+		return execCmd
+	}
 	if goos == runtime.GOOS && goarch == runtime.GOARCH {
 		return execCmd
 	}
@@ -751,6 +732,13 @@ func findExecCmd() []string {
 	return execCmd
 }
 
+func findGoCmd() string {
+	if *target != "" {
+		return "go_" + *target
+	}
+	return "go"
+}
+
 func (t *test) String() string {
 	return filepath.Join(t.dir, t.gofile)
 }