We need to find a better way to handle alloca on AIX.

See the discussion in the PR for https://bugs.openjdk.org/browse/JDK-8329257, e.g. https://github.com/openjdk/jdk/pull/18536#discussion_r1568650313 in which three alternatives are suggested. Quoting:

Let me summarize the choices we have and ask for your vote.
Magnus dislikes the -Dalloca'(size)'=__builtin_alloca'(size)' in flags-cflags.m4 I introduced to get rid of

#if defined(_AIX)
#include <alloca.h>
#endif

in globalDefinitions_gcc.hpp.

We have four possible solutions

1. Reintroduce

#if defined(_AIX)
#include <alloca.h>
#endif

in globalDefinitions_gcc.hpp.

2. Unconditionally introduce only #include <alloca.h> in globalDefinitions_gcc.hpp. This should work for all platforms using this header including the unofficial Windows/gcc Port, although only AIX needs it.

3. Add

#if defined(_AIX)
#include <alloca.h>
#endif

to the sources using alloca(). These are
/hotspot/share/runtime/os.cpp
/hotspot/share/runtime/javaThread.cpp
/hotspot/share/utilities/vmError.cpp
Here we need the AIX condition, because otherwise the classic Windows Build (NTAMD64) fails.

4. Replace -Dalloca'(size)'=__builtin_alloca'(size)' in flags-cflags.m4 by -U__STRICT_ANSI__ at the same place. Explanation can also found in https://github.com/openjdk/jdk/pull/18536#discussion_r1583360569 and following.

I will implement the solution with the most likes and having no dislike.