[compiler-rt][cmake] Use `GNUInstallDirs` to support custom installation dirs

[Ericson2314]

Summary

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

[Ericson2314]

N.B. I skipped review to land because the same code was approved in the original D99484. The pre-merge failure is unrelated, and I will be watchful to revert this evening if anything goes wrong with the full CI like the original bigger patch containing it.

I started with this one because it was quite self-contained.

compiler-rt/cmake/base-config-ix.cmake

 # The CompilerRT build system requires CMake version 2.8.8 or higher in order
 # to use its support for building convenience "libraries" as a collection of
 # .o files. This is particularly useful in producing larger, more complex
 # runtime libraries.
 
 include(CheckIncludeFile)
 include(CheckCXXSourceCompiles)
+include(GNUInstallDirs)
 include(ExtendPath)
 
 check_include_file(unwind.h HAVE_UNWIND_H)
 
 # Used by sanitizer_common and tests.
 check_include_file(rpc/xdr.h HAVE_RPC_XDR_H)
 if (NOT HAVE_RPC_XDR_H)
   set(HAVE_RPC_XDR_H 0)
set(COMPILER_RT_INSTALL_LIBRARY_DIR "${default_install_path}" CACHE PATH
     "Path where built compiler-rt libraries should be installed.")
 else(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
   set(COMPILER_RT_OUTPUT_LIBRARY_DIR
     ${COMPILER_RT_OUTPUT_DIR}/lib/${COMPILER_RT_OS_DIR})
   extend_path(default_install_path "${COMPILER_RT_INSTALL_PATH}" "lib/${COMPILER_RT_OS_DIR}")
   set(COMPILER_RT_INSTALL_LIBRARY_DIR "${default_install_path}" CACHE PATH
     "Path where built compiler-rt libraries should be installed.")
 endif()
-extend_path(default_install_path "${COMPILER_RT_INSTALL_PATH}" bin)
+extend_path(default_install_path "${COMPILER_RT_INSTALL_PATH}" "${CMAKE_INSTALL_BINDIR}")
 set(COMPILER_RT_INSTALL_BINARY_DIR "${default_install_path}" CACHE PATH
   "Path where built compiler-rt executables should be installed.")
-extend_path(default_install_path "${COMPILER_RT_INSTALL_PATH}" include)
+extend_path(default_install_path "${COMPILER_RT_INSTALL_PATH}" "${CMAKE_INSTALL_INCLUDEDIR}")
 set(COMPILER_RT_INSTALL_INCLUDE_DIR "${default_install_path}" CACHE PATH
   "Path where compiler-rt headers should be installed.")
-extend_path(default_install_path "${COMPILER_RT_INSTALL_PATH}" share)
+extend_path(default_install_path "${COMPILER_RT_INSTALL_PATH}" "${CMAKE_INSTALL_DATADIR}")
 set(COMPILER_RT_INSTALL_DATA_DIR "${default_install_path}" CACHE PATH
   "Path where compiler-rt data files should be installed.")
 
 if(APPLE)
   # On Darwin if /usr/include/c++ doesn't exist, the user probably has Xcode but
   # not the command line tools (or is using macOS 10.14 or newer). If this is
   # the case, we need to find the OS X sysroot to pass to clang.
   if(NOT EXISTS /usr/include/c++)