Set '-target' flag in the test checking the MacOS include dir

To fix a buildbot failure on PS4, see
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/42251

The test was added in r351222 and aims to check only a particular
Mac configuration. However it relied on the default compiler target
by default, therefore unintentionally failing on PS4.

llvm-svn: 351334