The _rotl method is unavailable on any platform but Windows,
so we can implement the method ourselves.

BUG=angle:773

Change-Id: I1342f4cf8a996daf11a36553c3bf7a01a2b182b3
Reviewed-on: https://chromium-review.googlesource.com/238500Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>