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

[coleen.phillimore at oracle.com]

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