RFR: JDK-8307349: Support xlc17 clang toolchain on AIX

[JoKern65]

The new xlc17 compiler should be supported to build OpenJDK on AIX. This compiler, compared to the currently supported xlc16, has a significantly more recent clang (xlc 17.1.1 uses clang 15) included.
1. Because the frontend interface of the new compiler (c-flags, Ld-Flags) has changed from an xlc to a clang interface we decided to use the clang toolchain for the new xlc17 compiler.
2. Unfortunately, the system headers are mainly unchanged, so they do not harmonize with the src/hotspot/share/utilities/globalDefinitions_gcc.hpp which would be used if we totally switch to clang toolchain. So we keep the HOTSPOT_TOOLCHAIN_TYPE=xlc
3. In src/hotspot/share/utilities/globalDefinitions_xlc.hpp we introduce a new define AIX_XLC_GE_17 which is set if we build with the new xlc17 on AIX. This define will be used in following PRs.

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Commit messages:
 - JDK-8307349

Changes: https://git.openjdk.org/jdk/pull/13898/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13898&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307349
  Stats: 117 lines in 6 files changed: 97 ins; 1 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/13898.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13898/head:pull/13898

PR: https://git.openjdk.org/jdk/pull/13898