RFR: 8375045: Incorrect Interpretation of POSIX TZ Environment Variable on AIX.

[Sruthy Jayan]

Bug Reference : https://bugs.openjdk.org/browse/JDK-8375045

On AIX, the POSIX TZ environment variable is not interpreted as expected when it includes DST rules.
Example: `TZ=CET-1CEST,M3.5.0,M10.5.0`

The fix includes : 
The TZ environment variable is processed by first matching the full value against tzmappings, then retrying with the value truncated at the first comma if no match is found, and finally defaulting to GMT if no mapping exists.

This fixes the regression caused by https://bugs.openjdk.org/browse/JDK-8292899

Signed-off-by: Sruthy Jayan <srutjay1 at in.ibm.com>

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Commit messages:
 - Fix to handle Incorrect Interpretation of POSIX TZ Environment Variable on AIX

Changes: https://git.openjdk.org/jdk/pull/29183/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29183&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375045
  Stats: 61 lines in 1 file changed: 39 ins; 18 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/29183.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29183/head:pull/29183

PR: https://git.openjdk.org/jdk/pull/29183