Switch from using the benchmark C++ API to our iree/testing/benchmark.h API and flags.

[benvanik]

benchmark's default flag handling doesn't work well for when a project also has their own handling. As part of the work to get our benchmarks running on bare metal we now have a (mostly complete) mapping of their API in C at iree/testing/benchmark.h. As part of this we can use the new flags support to have our own versions of the benchmark library flags that we use to provide consistent flags across all target devices whether using benchmark or not. It'll also let us use our flag verification instead of having to do UNDEFOK and ignoring typos in flags.

There's a TODO here:
https://github.com/google/iree/blob/cb5434134c0dfac50d7de4a1a965b5fe6d484c2d/iree/hal/local/executable_library_benchmark.c#L388-L394

and then iree-benchmark-module (and a few other files) should be switched to use the C API and custom flags parsing.