RFR: JDK-8329257: AIX: Switch HOTSPOT_TOOLCHAIN_TYPE from xlc to gcc [v6]

[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_gcc.hpp the compiler is much more nagging about ill formatted printf

Joachim Kern has updated the pull request incrementally with one additional commit since the last revision:

  replaced pragma alloca and include alloca by compiler define

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18536/files
  - new: https://git.openjdk.org/jdk/pull/18536/files/302ea6a7..801cfb54

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18536&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18536&range=04-05

  Stats: 8 lines in 3 files changed: 0 ins; 7 del; 1 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