[bolt][llvm][cmake] Use `CMAKE_INSTALL_LIBDIR` too

[Ericson2314]

Summary

Working back towards D130586.

Bolt didn't use LLVM_LIBDIR_SUFFIX before, and has no in-tree reverse
dependencies, it seems easier to add.

The change in LLVM itself is to prevent some unexpected lib64 from
cropping up due to the CMAKE_INSTALL_LIBDIR defaulting logic.

[Ericson2314]

This is a subset of D130586, which was approved, and unlike the original it appears to be passing CI all the way. I'll just go ahead and merge then so I can continue "bisecting".

bolt/runtime/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 include(CheckIncludeFiles)
+include(GNUInstallDirs)
 
 set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_CXX_STANDARD 17)
 
 project(libbolt_rt_project)
 
 check_include_files(elf.h HAVE_ELF_H)
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/config.h.in
set(BOLT_RT_FLAGS
   -mno-sse)
 
 # Don't let the compiler think it can create calls to standard libs
 target_compile_options(bolt_rt_instr PRIVATE ${BOLT_RT_FLAGS} -fPIE)
 target_include_directories(bolt_rt_instr PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
 target_compile_options(bolt_rt_hugify PRIVATE ${BOLT_RT_FLAGS})
 target_include_directories(bolt_rt_hugify PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
 
-install(TARGETS bolt_rt_instr DESTINATION lib)
-install(TARGETS bolt_rt_hugify DESTINATION lib)
+install(TARGETS bolt_rt_instr DESTINATION "${CMAKE_INSTALL_LIBDIR}")
+install(TARGETS bolt_rt_hugify DESTINATION "${CMAKE_INSTALL_LIBDIR}")
 
 if (CMAKE_CXX_COMPILER_ID MATCHES ".*Clang.*")
   add_library(bolt_rt_instr_osx STATIC
     instr.cpp
     ${CMAKE_CURRENT_BINARY_DIR}/config.h
   )
   target_include_directories(bolt_rt_instr_osx PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
   target_compile_options(bolt_rt_instr_osx PRIVATE
     -target x86_64-apple-darwin19.6.0
     ${BOLT_RT_FLAGS})
-  install(TARGETS bolt_rt_instr_osx DESTINATION lib)
+  install(TARGETS bolt_rt_instr_osx DESTINATION "${CMAKE_INSTALL_LIBDIR}")
 endif()

llvm/CMakeLists.txt

if (CMAKE_GENERATOR STREQUAL "Xcode" AND NOT CMAKE_OSX_ARCHITECTURES)
   # explicitly specify an architecture setting with the Xcode generator.
   set(CMAKE_OSX_ARCHITECTURES "x86_64")
 endif()
 
 project(LLVM
   VERSION ${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}
   LANGUAGES C CXX ASM)
 
-# Must go after project(..)
+if (NOT DEFINED CMAKE_INSTALL_LIBDIR AND DEFINED LLVM_LIBDIR_SUFFIX)
+  # Must go before `include(GNUInstallDirs)`.
+  set(CMAKE_INSTALL_LIBDIR "lib${LLVM_LIBDIR_SUFFIX}")
+endif()
+
+# Must go after `DEFINED LLVM_LIBDIR_SUFFIX` check.
+set(LLVM_LIBDIR_SUFFIX "" CACHE STRING "Define suffix of library directory name (32/64)" )
+
+# Must go after `project(..)`.
 include(GNUInstallDirs)
 
 # This C++ standard is required to build LLVM.
 set(LLVM_REQUIRED_CXX_STANDARD 17)
 
 # If we find that the cache contains CMAKE_CXX_STANDARD it means that it's a old CMakeCache.txt
 # and we can just inform the user and then reset it.
 if($CACHE{CMAKE_CXX_STANDARD} AND $CACHE{CMAKE_CXX_STANDARD} LESS ${LLVM_REQUIRED_CXX_STANDARD})

 
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 
 if (CMAKE_BUILD_TYPE AND
     NOT uppercase_CMAKE_BUILD_TYPE MATCHES "^(DEBUG|RELEASE|RELWITHDEBINFO|MINSIZEREL)$")
   message(FATAL_ERROR "Invalid value for CMAKE_BUILD_TYPE: ${CMAKE_BUILD_TYPE}")
 endif()
 
-set(LLVM_LIBDIR_SUFFIX "" CACHE STRING "Define suffix of library directory name (32/64)" )
-
 # LLVM_INSTALL_PACKAGE_DIR needs to be declared prior to adding the tools
 # subdirectory in order to have the value available for llvm-config.
 include(GNUInstallPackageDir)
 set(LLVM_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_PACKAGEDIR}/llvm" CACHE STRING
   "Path for CMake subdirectory for LLVM (defaults to '${CMAKE_INSTALL_PACKAGEDIR}/llvm')")
 
 set(LLVM_TOOLS_INSTALL_DIR "${CMAKE_INSTALL_BINDIR}" CACHE STRING
     "Path for binary subdirectory (defaults to '${CMAKE_INSTALL_BINDIR}')")