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 d178f4fc8 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