parser: add VER, REL and SUBDIR override for Retro targets

[Cyanoxygen]

This is useful for retro-specific packages, such as Mesa (mesa-amber) and librsvg (pre-Rust version).

[what-the-diff]

PR Summary

• Enhancement of 'release' variable retrieval
  We've updated how our system fetches the value of release. This now takes advantage of the get_var_arch function, instead of directly accessing the spec_var object.

• Modified method for extracting 'version'
  The version variable is now secured utilizing get_var_arch, as opposed to using the spec_var.get function earlier.

• Revised method to set 'subdir' variable
  The way we're setting the subdir variable has been changed. It is now being managed through the get_var method instead of the previous method of direct access from the spec_var.

These changes ensure better maintainability and reliability of how the system retrieves and handles these variables, promoting consistency in our code.

[Cyanoxygen]

Tested good with arch. specific packages and normal packages on ia64 and i486:

amd64: