unix: Provide "fast" target to build interpreter for benchmarking.

This build is primarily intended for benchmarking, and may have random
features enabled/disabled to get high scores in synthetic benchmarks.
The intent is to show/prove that MicroPython codebase can compete with
CPython, when configured appropriately. But the main MicroPython aim
still remains to optimize for memory usage (which inevitibly leads to
performance degradation in some areas on some workloads).