[mlir-cpu-runner] Add support for `-mattr` and `-march` flags

This patch adds support for `-mattr` and `-march` in mlir-cpu-runner.
With this change, one should be able to consistently use mlir-cpu-runner
for MLIR's integration tests (instead of e.g. resorting to lli when some
additional flags are needed). This is demonstrated in
concatenate_dim_1.mlir.

In order to support the new flags, this patch makes sure that
MLIR's ExecutionEngine/JITRunner (that mlir-cpu-runner is built on top of):
  * takes into account the new command line flags when creating
    TargetMachine,
  * avoids recreating TargetMachine if one is already available,
  * creates LLVM's DataLayout based on the previously configured
    TargetMachine.
This is necessary in order to make sure that the command line
configuration is propagated correctly to the backend code generator.

A few additional updates are made in order to facilitate this change,
including support for debug dumps from JITRunner.

Differential Revision: https://reviews.llvm.org/D146917