Enable a default native ARCH

uses a posix compatible script to find the native arch.
(based on ppigazzini's https://github.com/ppigazzini/stockfish-downloader )
use that native arch by default, no changes if ARCH is specified explicitly.

SF can now be compiled in an optimal way simply using

make -j profile-build

closes https://github.com/official-stockfish/Stockfish/pull/4777

No functional change