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

[JoKern65]

On Tue, 30 May 2023 21:44:17 GMT, JoKern65 <duke at openjdk.org> wrote:

>> 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?

At least the whole jdk actually builds with `#include <alloca.h>` in globalDefinitions_xlc.hpp

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PR Review Comment: https://git.openjdk.org/jdk/pull/14146#discussion_r1211272616