Do not build Stockfish for all targets by default

[niklasf]

build.rs builds optimized Stockfish and Fairy-Stockfish binaries for a variety of target CPUs, so that they can all be bundled in a portable binary. That's a lot of wasted build time, when building from source for a particular target machine, or during development.

Idea: By default, build only the best Stockfish for CFG_TARGET_FEATUERE. Add a new feature flag (e.g. portable) that builds all CPUs, to be used for official releases.

Complication: For a target supporting bmi2, we should probably build both avx2 and bmi2, to be selected at runtime, until fast bmi2 can be detected at compile time.