[11u] RFR: 8250636: iso8601_time returns incorrect offset part on MacOS

[Hohensee, Paul]

Companion request to 8u backport request.

JBS issue: https://bugs.openjdk.java.net/browse/JDK-8250636
Patch: https://hg.openjdk.java.net/jdk/jdk/rev/6103df4b7702

Backport webrev: http://cr.openjdk.java.net/~phh/8250636/webrev.11u.00/

Backport not clean due to having to update copyright date to 2020, and a context difference before the deletion of get_timezone().

Paired with backport of

JBS issue: https://bugs.openjdk.java.net/browse/JDK-8251365
Patch: https://hg.openjdk.java.net/jdk/jdk/rev/1096ad4dbf62

This patch applies cleanly after the above one for 8250636.

Thanks,
Paul