RFR: JDK-8307349: Support xlc17 clang toolchain on AIX [v2]

[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.

JoKern65 has updated the pull request incrementally with two additional commits since the last revision:

 - revert accidantially changed mode bits
 - I followed the proposals

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13898/files
  - new: https://git.openjdk.org/jdk/pull/13898/files/a94aa615..74331b8f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13898&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13898&range=00-01

  Stats: 12 lines in 5 files changed: 1 ins; 0 del; 11 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