Skip TestTargetXMLArch on non-darwin OSs

This test uses XML packets, but libxml is an optional dependency of
lldb, and this test fails if it is not present.

I'm leaving this enabled on mac, as thats the only platform that's
likely to have libxml always available, but ideally we should have a way
to skip this based on build configuration. I'll see if I can whip
something like that up soon, but for the time being, this unblocks the
buildbots.

llvm-svn: 324870