IBMClang: Some of CMake's tests still fail

IBMClang compiler support was added by !6785 (merged) for #22929 (closed). However, there are still a few failures in nightly testing on p8-aix2-llvm.osuosl.org.

They need further investigation:

RunCMake.GenerateExportHeader: Somehow the -fvisibility=hidden flag is being used, resulting in
```
.orig: error: unsupported option '-fvisibility=hidden' for target 'powerpc-ibm-aix7.2.0.0'
```

kwsys.testSystemTools: Fails with error:

TestFileAccess incorrectly indicated that this is a writable file:/.../Source/kwsys/testSystemToolsNewDir/testNewFile.txt

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