build: make nacl pass

Add nacl.bash, the NaCl version of all.bash.
It's a separate script because it builds a variant of package syscall
with a large zip file embedded in it, containing all the input files
needed for tests.

Disable various tests new since the last round, mostly the ones using os/exec.

Fixes #7945.

LGTM=dave
R=golang-codereviews, remyoudompheng, dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/100590044
diff --git a/src/pkg/runtime/crash_test.go b/src/pkg/runtime/crash_test.go
index dbcd948..b2e846a 100644
--- a/src/pkg/runtime/crash_test.go
+++ b/src/pkg/runtime/crash_test.go
@@ -9,6 +9,7 @@
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 	"text/template"
@@ -31,6 +32,10 @@
 }
 
 func executeTest(t *testing.T, templ string, data interface{}) string {
+	if runtime.GOOS == "nacl" {
+		t.Skip("skipping on nacl")
+	}
+
 	checkStaleRuntime(t)
 
 	st := template.Must(template.New("crashSource").Parse(templ))