[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
1 file changed
tree: 6cf9f612d3185ca5e30fceb4b979efe2d2473f21
  1. cmake/
  2. docs/
  3. include/
  4. src/
  5. test/
  6. .clang-format
  7. CMakeLists.txt
  8. LICENSE.TXT