lib.systems: correct and simplify canExecute condition regarding march (#479638)

            # assume compatible cpu have all the instructions included

            final.parsed.cpu == platform.parsed.cpu

            ->

              # if both have gcc.arch defined, check whether final can execute the given platform

              # if platform has gcc.arch, final must also have and can execute the gcc.arch of platform

              (

                (final ? gcc.arch && platform ? gcc.arch)

                -> architectures.canExecute final.gcc.arch platform.gcc.arch

                platform ? gcc.arch -> final ? gcc.arch && architectures.canExecute final.gcc.arch platform.gcc.arch

              )

              # if platform has gcc.arch defined but final doesn't, don't assume it can be executed

              || (platform ? gcc.arch -> !(final ? gcc.arch))

          );

        isCompatible =