RFR: 8329257: AIX: Switch HOTSPOT_TOOLCHAIN_TYPE from xlc to gcc [v3]

[Kim Barrett]

On Tue, 30 Apr 2024 15:19:47 GMT, Joachim Kern <jkern at openjdk.org> wrote:

>> It might be easier to get input if you create a new PR with the change. This discussion is hidden deep down in a closed PR.
>
> I will do after labor day and create a PR with this suggested solution in your JDK-8330539.

I think I still prefer just unconditionally including <alloca.h> in globalDefinitions_gcc.hpp.  For gcc/clang we are using `-std=c++14` + `-D_GNU_SOURCE` instead of `-std=gnu++14`.  I forget exactly why.  I don't really want
to be messing with `__STRICT_ANSI__`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18536#discussion_r1585181094