diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -77,6 +77,16 @@
 
   set_enable_per_target_runtime_dir()
 
+  get_cmake_property(variableNames VARIABLES)
+  foreach(variableName ${variableNames})
+    string(FIND "${variableName}" "BUILTINS_default" out)
+    if("${out}" EQUAL 0)
+      string(REPLACE "BUILTINS_default_" "" new_name ${variableName})
+      string(REPLACE ";" "|" new_value "${${variableName}}")
+      list(APPEND default_target_extra_args "-D${new_name}=${new_value}")
+    endif()
+  endforeach()
+
   llvm_ExternalProject_Add(builtins
                            ${compiler_rt_path}/lib/builtins
                            DEPENDS ${ARG_DEPENDS}
@@ -88,6 +98,7 @@
                                       -DCMAKE_ASM_COMPILER_WORKS=ON
                                       ${COMMON_CMAKE_ARGS}
                                       ${BUILTINS_CMAKE_ARGS}
+                                      ${default_target_extra_args}
                            PASSTHROUGH_PREFIXES COMPILER_RT
                                                 DARWIN
                                                 SANITIZER
@@ -214,6 +225,17 @@
     endif()
   endforeach()
 
+  set(default_target_extra_args ${ARG_CMAKE_ARGS})
+  get_cmake_property(variableNames VARIABLES)
+  foreach(variableName ${variableNames})
+    string(FIND "${variableName}" "RUNTIMES_default_" out)
+    if("${out}" EQUAL 0)
+      string(REPLACE "RUNTIMES_default_" "" new_name ${variableName})
+      string(REPLACE ";" "|" new_value "${${variableName}}")
+      list(APPEND default_target_extra_args "-D${new_name}=${new_value}")
+    endif()
+  endforeach()
+
   if(LLVM_INCLUDE_TESTS)
     set_property(GLOBAL APPEND PROPERTY LLVM_ALL_LIT_TESTSUITES "@${LLVM_BINARY_DIR}/runtimes/runtimes-bins/lit.tests")
     list(APPEND test_targets runtimes-test-depends check-runtimes)
@@ -236,7 +258,7 @@
                                       -DCMAKE_ASM_COMPILER_WORKS=ON
                                       ${COMMON_CMAKE_ARGS}
                                       ${RUNTIMES_CMAKE_ARGS}
-                                      ${ARG_CMAKE_ARGS}
+                                      ${default_target_extra_args}
                            PASSTHROUGH_PREFIXES LLVM_ENABLE_RUNTIMES
                                                 LLVM_USE_LINKER
                                                 ${ARG_PREFIXES}