IBMClang: Do not use -fvisibility on AIX

Use of `-fvisibility=hidden` fails:

  unsupported option '-fvisibility=hidden' for target 'powerpc-ibm-aix7.2.0.0'

Apply the change from commit 4feba34d02 (GNU: Do not use -fvisibility on
AIX or HP-UX, 2016-09-03, v3.7.0-rc1~173^2~2) to IBMClang also.

Issue: #23157