[build/clang_darwin] Fix the case reversal in r173465.

 - I got confused by the double negative, the test succends on 10.6, in which
   case we *do* want to perform the filter-out.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173977 91177308-0d34-0410-b5e6-96231b3b80d8