mojo: Add a runloop utility in python

mojo/public/python/BUILD.gn

Index: mojo/public/python/BUILD.gn
diff --git a/mojo/public/python/BUILD.gn b/mojo/public/python/BUILD.gn
index 73653f76000d9b9cddba96f34ec8405f4367d40f..b2e6e0aed49302ec5239536bb38d4b51405d0102 100644
--- a/mojo/public/python/BUILD.gn
+++ b/mojo/public/python/BUILD.gn
@@ -16,10 +16,16 @@ python_binary_module("system") {
   python_base_module = "mojo"
   sources = [
     "mojo/c_core.pxd",
+    "mojo/c_environment.pxd",
     "mojo/system.pyx",
   ]
+  additional_sources = [
+    "src/python_system_helper.cc",
+    "src/python_system_helper.h",
+  ]
   deps = [
     "//mojo/public/c/system",
+    "//mojo/public/cpp/utility",
     ":base",
   ]
 }