child_process: use /system/bin/sh on android

`/bin/sh` does not exist on Android but `/system/bin/sh` does.

PR-URL: https://github.com/nodejs/node/pull/6745
Refs: https://github.com/nodejs/node/pull/6733
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

lib/child_process.js

@@ -332,7 +332,12 @@ function normalizeSpawnArguments(file /*, args, options*/) {
       args = ['/s', '/c', '"' + command + '"'];
       options.windowsVerbatimArguments = true;
     } else {
-      file = typeof options.shell === 'string' ? options.shell : '/bin/sh';
+      if (typeof options.shell === 'string')
+        file = options.shell;
+      else if (process.platform === 'android')
+        file = '/system/bin/sh';
+      else
+        file = '/bin/sh';
       args = ['-c', command];
     }
   }