After submitting the DataLayout fix, some tests fail when they didn't
before. This has to do with the target essentially being ignored when
these tests were run earlier, as the --target x86-unknown-linux-gnu
only has to be correctly formed to be accepted.
Now the target triple is actually being used to get the targetmachine
earlier - before MLIR is generated - so the test that has a valid target
but not available on the platform fails.
Fix is to require x86 registered target when running those tests.