Fix `check-mlir` target when the host target isn't configured

This patch adds the `default_triple` feature to MLIR test suite.
This feature was added to LLVM in d178f4fc in order to be able to
run the LLVM tests without having the host targets configured in.
With this change, `ninja check-mlir` passes without the host
target, i.e. this config:

  cmake ../llvm -DLLVM_TARGETS_TO_BUILD="" -DLLVM_DEFAULT_TARGET_TRIPLE="" -DLLVM_ENABLE_PROJECTS=mlir -GNinja

Differential Revision: https://reviews.llvm.org/D82142