RFR: 8250636: iso8601_time returns incorrect offset part on MacOS

[Dmitry Cherepanov]

Hello,

Please review a small change for fixing offset (timezone) part of the
string returned by os::iso8601_time on MacOS. The patch negates
tm_gmtoff and skips the DST adjustment when tm_gmtoff is used. The
behavior on other platforms should remain unchanged. More details are in
the bug report.

JBS: https://bugs.openjdk.java.net/browse/JDK-8250636
Webrev: http://cr.openjdk.java.net/~dcherepanov/8250636/webrev.v3/

Thanks,

Dmitry