[libc] Support multilib with runtimes build (llvm#115357)

This adds minimal support for multilibs akin to libc++.

Author: petrhosek

libc/CMakeLists.txt

@@ -248,10 +248,17 @@ foreach(config_path IN LISTS LIBC_CONFIG_JSON_FILE_LIST)
   load_libc_config(${config_path}/config.json ${cmd_line_conf})
 endforeach()
 
+if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
+  set(LIBC_TARGET_SUBDIR ${LLVM_DEFAULT_TARGET_TRIPLE})
+  if(LIBC_LIBDIR_SUBDIR)
+    string(APPEND LIBC_TARGET_SUBDIR /${LIBC_LIBDIR_SUBDIR})
+  endif()
+endif()
+
 if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND (LIBC_ENABLE_USE_BY_CLANG OR LIBC_TARGET_OS_IS_GPU))
   set(LIBC_INCLUDE_DIR ${LLVM_BINARY_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE})
   set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
-  set(LIBC_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
+  set(LIBC_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LIBC_TARGET_SUBDIR})
 else()
   if(NOT LIBC_ENABLE_USE_BY_CLANG)
     set(LIBC_INCLUDE_DIR ${CMAKE_CURRENT_BINARY_DIR}/include)
@@ -277,8 +284,7 @@ endif()
 if(LIBC_TARGET_TRIPLE)
   set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LIBC_TARGET_TRIPLE})
 elseif(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
-  set(LIBC_INSTALL_LIBRARY_DIR
-      lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE})
+  set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LIBC_TARGET_SUBDIR})
 else()
   set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX})
 endif()

llvm/runtimes/CMakeLists.txt

@@ -635,6 +635,7 @@ if(build_runtimes)
           CMAKE_ARGS -DLLVM_DEFAULT_TARGET_TRIPLE=${name}
                      -DLLVM_RUNTIMES_PREFIX=${name}/
                      -DLLVM_RUNTIMES_LIBDIR_SUBDIR=${multilib}
+                     ${extra_cmake_args}
           BASE_NAME ${name}
           EXTRA_ARGS TARGET_TRIPLE ${name})
       endforeach()