def build_arch(self, arch):
        """
        Creates expected build and symlinks system Python version.
        """
        self.ctx.hostpython = '/usr/bin/false'
        # creates the sub buildir (used by other recipes)
        # https://github.com/kivy/python-for-android/issues/1154
        sub_build_dir = join(self.get_build_dir(), 'build')
        shprint(sh.mkdir, '-p', sub_build_dir)
        python3crystax = self.get_recipe('python3crystax', self.ctx)
        system_python = sh.which("python" + python3crystax.version)
        if system_python is None:
            raise OSError(
                ('Trying to use python3crystax=={} but this Python version '
                 'is not installed locally.').format(python3crystax.version))
        link_dest = join(self.get_build_dir(), 'hostpython')
        shprint(sh.ln, '-sf', system_python, link_dest)
Creates expected build and symlinks system Python version.