XFAIL the Mips tests when we don't have the target.

These tests shouldn't even try to use the Mips target, but let's make
them not fail when we don't have it while they don't get fixed.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@206947 91177308-0d34-0410-b5e6-96231b3b80d8