[build/clang_darwin] Don't try to run sw_vers if we don't have it.
 - Also, fix the test this is guarding, which was backwards.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173465 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed