XL: Recognize compilers identified by __ibmxl__

Brad Kingrequested to merge
brad.king/cmake:xl-new-compiler-macros into master

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 (closed)

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