Check ARCH=.... variable

to prevent user errors or generating untested code,
check explicitly that the ARCH variable is equivalent to a supported architecture
as listed in `make help`.

To nevertheless compile for an untested target the user can override the internal
variable, passing the undocumented `SUPPORTED_ARCH=true` to make.

closes https://github.com/official-stockfish/Stockfish/pull/3062

No functional change.