RFR: 8166117: Add UTC timestamp decorator for UL

[Marcus Larsson]

Hi,

Please review the following patch to add a UTC timestamp decorator for UL.

os::iso8601_time() has been modified to allow timestamps based on UTC. 
os::gmtime_pd() has been added to replace os::localtime_pd() when UTC is 
requested. Patch also includes a unit test for the new decoration.

Webrev:
http://cr.openjdk.java.net/~mlarsson/8166117/webrev.00/

Issue:
https://bugs.openjdk.java.net/browse/JDK-8166117

Testing:
New unit test through JPRT.

Thanks,
Marcus