[mlir] Use 'native' instead of 'llvm_has_native_target' in the mlir tests

The tests actually require the target triple to match the host, rather than just having the host in the list of available targets. This change removes `llvm_has_native_target` and instead uses the `native` feature from the lit configuration.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D126011