Index: runtimes/CMakeLists.txt
===================================================================
--- runtimes/CMakeLists.txt
+++ runtimes/CMakeLists.txt
@@ -58,6 +58,8 @@
   # is required because the other runtimes need the builtin libraries present
   # before the just-built compiler can pass the configuration tests.
   if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt)
+    string(REPLACE "-" ";" TARGET_TRIPLE_LIST ${TARGET_TRIPLE})
+    list(GET TARGET_TRIPLE_LIST 0 COMPILER_RT_DEFAULT_TARGET_ARCH)
     llvm_ExternalProject_Add(builtins
                              ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt/lib/builtins
                              PASSTHROUGH_PREFIXES COMPILER_RT