Isolated emscripten in new lib

lib/emscripten/emtests/test_runtime_stacksave.c

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2017 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <emscripten.h>
+#include <assert.h>
+
+int main() {
+  int x = EM_ASM_INT({ return stackSave(); });
+  int y = EM_ASM_INT({ return stackSave(); });
+  EM_ASM_INT({ out($0); }, &x);
+  EM_ASM_INT({ out($0); }, &y);
+  assert(x == y);
+  EM_ASM({ out('success'); });
+}