docs: 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)