[flang]Fix build failure in tests

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.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D137335