Condition the use of UI64 on _MSC_VER.

Using OPENSSL_WINDOWS for this is inaccurate because it's really a
feature of the compiler, not the platform. I think it's only MSVC that
uses the UI64 suffix.

Change-Id: I4a95961b94e69e72b93f5ed1e0457661b74242c8
Reviewed-on: https://boringssl-review.googlesource.com/2730
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>