[CMake][libcxx] Use builtins rather than gcc_s when compiler-rt is requested

When compiler-rt is requested, we should attempt to link compiler-rt
builtins library rather than gcc_s.

Differential Revision: https://reviews.llvm.org/D31617

llvm-svn: 299599
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 963bcd2ec31fd512b11bb557ec0c73337867dbe4
diff --git a/cmake/Modules/HandleCompilerRT.cmake b/cmake/Modules/HandleCompilerRT.cmake
index 9730008..41cdd44 100644
--- a/cmake/Modules/HandleCompilerRT.cmake
+++ b/cmake/Modules/HandleCompilerRT.cmake
@@ -3,8 +3,11 @@
     message(FATAL_ERROR "LIBCXX_COMPILE_FLAGS must be defined when using this function")
   endif()
   set(dest "" PARENT_SCOPE)
-  set(CLANG_COMMAND ${CMAKE_CXX_COMPILER} ${LIBCXX_COMPILE_FLAGS}
+  set(CLANG_COMMAND ${CMAKE_CXX_COMPILER} ${TARGET_TRIPLE} ${LIBCXX_COMPILE_FLAGS}
       "--rtlib=compiler-rt" "--print-libgcc-file-name")
+  if (CMAKE_CXX_COMPILER_ID MATCHES Clang AND CMAKE_CXX_COMPILER_TARGET)
+    list(APPEND CLANG_COMMAND "--target=${CMAKE_CXX_COMPILER_TARGET}")
+  endif()
   execute_process(
       COMMAND ${CLANG_COMMAND}
       RESULT_VARIABLE HAD_ERROR