build/clang_darwin: Don't try to build ARM contents on 10.6.

 - The cctools' ranlib on 10.6 has a bug and can't understand our ARM object
   files.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@166114 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed