Unbreak microbenchmark build (#5710)
Minor updates to the microbenchmarks built optionally with "--build_micro_benchmarks". These are not built as part of CI, and builds started to fail. There are three changes:
- I updated the threading-related benchmarks to use the static-method ThreadPool API, and to expose control over the thread pool configuration via constexpr int variables.
- Disable GCC warnings seen with recent compiler versions when including parts of the Eigen headers in batchnorm.cc and eigen.cc files.
- Flush std::cerr on error conditions to avoid buffered messages being lost.
I tested manual builds with Linux (GCC) and Windows (MSVC).
