test: Fix feature test for "native" in lit.cfg

Checking if the host arch is in the triple isn't quite correct. Change
the feature test to match llvm's, which made the same change in r193459.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203540 91177308-0d34-0410-b5e6-96231b3b80d8