Don't use -fomit-frame-pointer on darwin, it breaks stacktrace collection.

llvm-svn: 28451
diff --git a/llvm/Makefile.rules b/llvm/Makefile.rules
index b3b10e5..96f2343 100644
--- a/llvm/Makefile.rules
+++ b/llvm/Makefile.rules
@@ -208,8 +208,11 @@
     BuildMode := Release
     # Don't use -fomit-frame-pointer on FreeBSD
     ifneq ($(OS),FreeBSD)
+    # Don't use -fomit-frame-pointer on Darwin, it breaks backtraces.
+    ifneq ($(OS),Darwin)
       OmitFramePointer := -fomit-frame-pointer
     endif
+    endif
     CXX.Flags := -O3 -finline-functions -felide-constructors $(OmitFramePointer)
     C.Flags   := -O3 $(OmitFramePointer)
     LD.Flags  := -O3