Un-revert "Support for multiarch runtimes layout" This reverts https://reviews.llvm.org/rL336005, which was an accidental commit.

CMakeLists.txt

 # Add path for custom modules
 set(CMAKE_MODULE_PATH
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
     ${CMAKE_MODULE_PATH})
 
 set(LIBUNWIND_COMPILER    ${CMAKE_CXX_COMPILER})
 set(LIBUNWIND_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBUNWIND_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
-if (LLVM_LIBRARY_OUTPUT_INTDIR)
+
+string(REGEX MATCH "[0-9]+\\.[0-9]+(\\.[0-9]+)?" CLANG_VERSION
+       ${PACKAGE_VERSION})
+
+if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
+  set(DEFAULT_INSTALL_PREFIX lib${LLVM_LIBDIR_SUFFIX}/clang/${CLANG_VERSION}/${LLVM_DEFAULT_TARGET_TRIPLE}/)
+  set(LIBUNWIND_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/clang/${CLANG_VERSION}/${LLVM_DEFAULT_TARGET_TRIPLE}/lib${LLVM_RUNTIMES_LIBDIR_SUFFIX})
+elseif(LLVM_LIBRARY_OUTPUT_INTDIR)
   set(LIBUNWIND_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
 else()
   set(LIBUNWIND_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBUNWIND_LIBDIR_SUFFIX})
 endif()
 
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${LIBUNWIND_LIBRARY_DIR})
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LIBUNWIND_LIBRARY_DIR})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LIBUNWIND_LIBRARY_DIR})
 
-set(LIBUNWIND_INSTALL_PREFIX "" CACHE STRING
+set(LIBUNWIND_INSTALL_PREFIX ${DEFAULT_INSTALL_PREFIX} CACHE STRING
     "Define libunwind destination prefix.")
 
-if (NOT LIBUNWIND_INSTALL_PREFIX MATCHES "^$|.*/")
-  message(FATAL_ERROR "LIBUNWIND_INSTALL_PREFIX has to end with \"/\".")
-endif()
-
 set(LIBUNWIND_C_FLAGS "")
 set(LIBUNWIND_CXX_FLAGS "")
 set(LIBUNWIND_COMPILE_FLAGS "")
 set(LIBUNWIND_LINK_FLAGS "")
 
 # Get required flags.
 macro(append_if list condition var)
   if (${condition})