Prefer `process.stdout.write` in xUnit reporter.

Falls back to `console.log` if `process.stdout` is unavailable.

This is another stab at fixing #1068 without breaking existing use cases. It works for me on my own ~1500 test suite in both Chrome and through mocha-phantomjs, and the Mocha self-tests pass.

I was having an issue with mocha-phantomjs not picking up the reporter output when used with the `--file` options because of the use of `console.log`. Switching to `process.stdout.write` (when it is available) fixes the problem for me.

This also seems to have picked up a completely unrelated change to another file that hadn't yet made it into the mocha.js snapshot. (See diff.) Is this sort of thing a regular occurrence?