[llvm-ar][test] Correct tests marked as expected fails

In diff D64802 I marked three tests as expected failures for darwin but
James Nagurne saw these fail on his downstream embedded ARM cross
compiler.
I believe XFAIL: system-darwin should be used instead of using XFAIL:
darwin due to the problem being related to the darwin host and not the
target.

Differential Revision: https://reviews.llvm.org/D65745

llvm-svn: 368564