[libcxx][libcxxabi][libunwind][cmake] Use `GNUInstallDirs` to support custom installation dirs

I am breaking apart D99484 so the cause of build failures is easier to
understand.

Differential Revision: https://reviews.llvm.org/D117417

GitOrigin-RevId: f16a4a034a279f52c33f41cafb7d9751ee8a01dd