RFR: JDK-8308288: Fix xlc17 clang warnings and build errors in hotspot

[JoKern65]

This pr is a split off from JDK-8308288 : Fix xlc17 clang warnings in shared code https://github.com/openjdk/jdk/pull/14146
It handles the part in hotspot.

It handles the error introduced by a redefine of malloc in stdlib.h resulting in the following build error:

/data/d042520/pr/jdk/src/hotspot/share/runtime/os.cpp:616:5: error: no member named '_vec_malloc' in 'LogTag'; did you mean 'vec_malloc'?
    log_warning(malloc, free)("ptr caught: " PTR_FORMAT, p2i(ptr));
    ^~~~~~~~~~~~~~~~~~~~~~~~~
/data/d042520/pr/jdk/src/hotspot/share/logging/log.hpp:46:28: note: expanded from macro 'log_warning'
#define log_warning(...) (!log_is_enabled(Warning, __VA_ARGS__)) ? (void)0 : LogImpl<LOG_TAGS(__VA_ARGS__)>::write<LogLevel::Warning>
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/data/d042520/pr/jdk/src/hotspot/share/logging/log.hpp:68:45: note: expanded from macro 'log_is_enabled'
#define log_is_enabled(level, ...) (LogImpl<LOG_TAGS(__VA_ARGS__)>::is_level(LogLevel::level))
                                            ^~~~~~~~~~~~~~~~~~~~~
/data/d042520/pr/jdk/src/hotspot/share/logging/logTag.hpp:221:38: note: expanded from macro 'LOG_TAGS'
#define LOG_TAGS(...) EXPAND_VARARGS(LOG_TAGS_EXPANDED(__VA_ARGS__, _NO_TAG, _NO_TAG, _NO_TAG, _NO_TAG, _NO_TAG, _NO_TAG))
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/data/d042520/pr/jdk/src/hotspot/share/logging/logTag.hpp:217:57: note: expanded from macro 'LOG_TAGS_EXPANDED'
#define LOG_TAGS_EXPANDED(T0, T1, T2, T3, T4, T5, ...)  PREFIX_LOG_TAG(T0), PREFIX_LOG_TAG(T1), PREFIX_LOG_TAG(T2), \
                                                        ^~~~~~~~~~~~~~~~~~
   ... (rest of output omitted)


Additionally it solves the need for an #include <alloca.h> on AIX for any usage of the alloca function, by adding the include to globalDefinitions_xlc.hpp

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

Commit messages:
 - JDK-8308288

Changes: https://git.openjdk.org/jdk/pull/14283/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14283&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308288
  Stats: 13 lines in 2 files changed: 10 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/14283.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14283/head:pull/14283

PR: https://git.openjdk.org/jdk/pull/14283