XL: Recognize compilers identified by __ibmxl__

IBM XL C/C++ for Linux versions 13.1.6 and above no longer define
`__IBMC__` or `__IBMCPP__` by default (see `-qxlcompatmacros`).
Instead `__ibmxl__` now identifies the compiler along with some
related new version macros.

Fixes: #17784
7 jobs for xl-new-compiler-macros
Status Name Job ID Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

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passed ghostflow-stager