Added a unit test for r173983 that verifies that Target.isiOS() works correctly.

As a bonus I put in some extra checks to make sure that we are identifying the
machine word of various Mac OS X/iOS targets appropriately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173994 91177308-0d34-0410-b5e6-96231b3b80d8