RFR (S) 8182554: Code for os::random() assumes long is 32 bits

[Kim Barrett]

> On Jun 28, 2017, at 11:46 AM, coleen.phillimore at oracle.com wrote:
> 
> Summary: And make updating the _rand_seed thread safe.
> 
> See bug for more details.
> 
> open webrev at http://cr.openjdk.java.net/~coleenp/8182554.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8182554
> 
> Tested with JPRT and performance tested addition of cas in os::random call (no regressions).   The only thing that uses os::random more than like once is Symbol creation.
> 
> Thanks,
> Coleen

Something I missed during pre-review.  Sorry!
- random_helper ought to be static (or added to os::).

Also, the seed in test_random is still (unsigned) long, and we're
trying to eliminate potentially confusing uses of long.

Looks good.  I don't need another webrev for those nits.