RFR: JDK-8329257: AIX: Switch HOTSPOT_TOOLCHAIN_TYPE from xlc to gcc

[Joachim Kern]

As of [JDK-8325880](https://bugs.openjdk.org/browse/JDK-8325880), building the JDK requires version 17 of IBM Open XL C/C++ (xlc). This is in effect clang by another name, and it uses the clang toolchain in the JDK build. Thus the old xlc toolchain was removed by [JDK-8327701](https://bugs.openjdk.org/browse/JDK-8327701).
Now we also switch the HOTSPOT_TOOLCHAIN_TYPE from xlc to gcc, removing the last xlc rudiment.
This means merging the AIX specific content of utilities/globalDefinitions_xlc.hpp and utilities/compilerWarnings_xlc.hpp into the corresponding gcc files on the on side and removing the defined(TARGET_COMPILER_xlc) blocks in the code, because the defined(TARGET_COMPILER_gcc) blocks work out of the box for the new AIX compiler.
The rest of the changes are needed because of using utilities/compilerWarnings_xlc.hpp the compiler is much more nagging about ill formatted printf

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Commit messages:
 - JDK-8329257

Changes: https://git.openjdk.org/jdk/pull/18536/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18536&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329257
  Stats: 261 lines in 13 files changed: 21 ins; 208 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/18536.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18536/head:pull/18536

PR: https://git.openjdk.org/jdk/pull/18536