Clean-up Makefile help

Clean-up Makefile help

Do not show the details of the default architecture for a simple "make help"
invocation, as the details are most likely to confuse beginners. Instead we
make it clear which architecture is the default and put an example at the end
of the Makefile as an incentative to use "make help ARCH=blah" to discover
the flags used by the different architectures.

```
    make help
    make help ARCH=x86-64-ssse3
```

Also clean-up and modernize a bit the Makefile examples while at it.

closes https://github.com/official-stockfish/Stockfish/pull/2996

No functional change