RFR: JDK-8304291: [AIX] Broken build after JDK-8301998

[Matthias Baesken]

After the latest harfbuzz update, the AIX build is broken.  The old clang compiler from xlc16 does not compile harfbuzz correctly.
First issue in hb-algs.hh is that xlc16 clang still sets some GNUC-related macros, so we do not run into the  `__clang_major__ >= 8`  check that should prevent to try to compile  `__builtin_mul_overflow`   with ancient clang.
The other issue in  `hb-subset.cc`  is a bit tricky and has been observed as well on macOS when very old clang versions were used.

Probably we can get rid of those 2 workarounds in some months after switching to xlc17 which includes a rather new clang version.

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Commit messages:
 - JDK-8304291

Changes: https://git.openjdk.org/jdk/pull/13529/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13529&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304291
  Stats: 6 lines in 2 files changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13529.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13529/head:pull/13529

PR: https://git.openjdk.org/jdk/pull/13529