RFR: JDK-8308288: Fix xlc17 clang warnings in shared code [v2]

[JoKern65]

On Tue, 30 May 2023 18:42:54 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> JoKern65 has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   forgotton _
>
> src/hotspot/share/runtime/javaThread.cpp line 115:
> 
>> 113: #include <alloca.h>
>> 114: #endif
>> 115: 
> 
> Could these conditionals be included in globalDefinitions_xlc.hpp instead?

In principle the `#include <alloca.h>` could be included in globalDefinitions_xlc.hpp. But alloca.h implements it with a
`#undef  alloca`
`#define alloca(size)   __builtin_alloca (size)`
If this new global define does not introduce new trouble (even in future coding) when it is seen in every source, we can go this way.
Are there any obstacles from anyone else?

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

PR Review Comment: https://git.openjdk.org/jdk/pull/14146#discussion_r1210853233