Re-apply 66315, but restrict it to Darwin only.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66427 91177308-0d34-0410-b5e6-96231b3b80d8

Makefile.rules

@@ -519,6 +519,16 @@ ifdef UNIVERSAL
 
   # Building universal cannot compute dependencies automatically.
   DISABLE_AUTO_DEPENDENCIES=1
+else
+  ifeq ($(OS),Darwin)
+    ifeq ($(ARCH),x86_64)
+      CompileCommonOpts += -m64
+    else
+      ifeq ($(ARCH),x86)
+        CompileCommonOpts += -m32
+      endif
+    endif
+  endif
 endif
 
 ifeq ($(OS),SunOS)