[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.
[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: c199f293c43da69278bea8e88f92242bf3aa95f7@173465 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt