Address Compiler Warnings when building on AIX with OpenXL

[midronij]

When building on AIX with OpenXL, the build will fail due a large number of unused variable/function errors such as the following:

/home/midronij/openj9-openjdk-jdk21/omr/port/aix/omrosbacktrace_impl.c:103:8: error: unused variable 'r1' [-Werror,-Wunused-variable]
  103 |         void *r1;
      |               ^~
/home/midronij/openj9-openjdk-jdk21/omr/port/aix/omrosbacktrace_impl.c:180:12: error: unused variable 'count' [-Werror,-Wunused-variable]
  180 |         uintptr_t count;
      |                   ^~~~~
2 errors generated.

To work around this, the -Werror flag will temporarily be omitted for builds done with OpenXL:

if(NOT CMAKE_C_COMPILER_IS_OPENXL)
	set(OMR_C_WARNINGS_AS_ERROR_FLAG -Werror)
	set(OMR_CXX_WARNINGS_AS_ERROR_FLAG -Werror)
	set(OMR_NASM_WARNINGS_AS_ERROR_FLAG -Werror)
endif()

(from cmake/modules/platform/toolcfg/gnu.cmake).

However, to remain consistent with the flags currently being used to build on AIX with xlC, these warnings/errors should be addressed so that the -Werror flag can be used without the build failing.