Failing jdk11u AIX build on Adoptium servers

[Tyler Steele]

Hi all,

In regards to the new Harfbuzz version bump (and the work done to prepare for it). And of special importance now that
the Adoptium build of jdk11u-dev is
failing [https://ci.adoptopenjdk.net/job/AIX-jdk11-dev-build/17/execution/node/51/log/], I wanted to suggest a course of
action to the AIX-folks here to get suggestions before I make any changes.

Early this week, I spent some time looking into this issue. My assessment is that AIX builds for jdk versions 11 and
greater will need the at least xlc version 16.1 to support the Harfbuzz change. This is because full support for C++11
with xlc did not arrive on AIX until that version (little-endian support was complete in 13.1).

For reference:
"IBM XL C++ for AIX
 - Core language support status: C++11 partial in 13.1.3 and 16.1.0 (xlC frontend), complete in 16.1.0 (xlclang
frontend)" [source: https://en.cppreference.com/w/cpp/compiler_support ]

My expected course of action is as follows:
- Bump the minimum requirements in our Supported Builds document
[https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms]
- Open an issue with the Adoptium group to change their build for jdk11 and above to use the version 16 compiler.

As I am still relatively new to AIX and OpenJDK development, so I value any suggestions for improvements, omissions you
may have noticed, or generally other comments the team here may have.

Tyler