crossfiles: Streamline and simplify crossfiles

* `needs_exe_wrapper` is only needed in specific cases when
  `exe_wrapper` is not set.
  See https://mesonbuild.com/Cross-compilation.html#properties

* "Before 0.56.0, <lang>_args and <lang>_link_args must be put in the
   properties section instead, else they will be ignored."
   [https://mesonbuild.com/Machine-files.html#meson-builtin-options]
   Our minimum version is 0.49.0. Meson >= 0.56.0 prints a deprecation
   warning.