Revert "Prepare Compiler-RT for GnuInstallDirs, matching libcxx"

This reverts commit 9a9bc76c.

That commit broke "ninja install" when building compiler-rt for mingw
targets, building standalone (pointing cmake at the compiler-rt
directory) with cmake 3.16.3 (the one shipped in ubuntu 20.04), with
errors like this:

-- Install configuration: "Release"
CMake Error at cmake_install.cmake:44 (file):
  file cannot create directory: /include/sanitizer.  Maybe need
  administrative privileges.
Call Stack (most recent call first):
  /home/martin/code/llvm-mingw/src/llvm-project/compiler-rt/build-i686-sanitizers/cmake_install.cmake:37 (include)

FAILED: include/CMakeFiles/install-compiler-rt-headers
cd /home/martin/code/llvm-mingw/src/llvm-project/compiler-rt/build-i686-sanitizers/include && /usr/bin/cmake -DCMAKE_INSTALL_COMPONENT="compiler-rt-headers" -P /home/martin/code/llvm-mingw/src/llvm-project/compiler-rt/build-i686-sanitizers/cmake_install.cmake
ninja: build stopped: subcommand failed.