[WIP] Add settings to toggle `%gp_rel` usage on generated assembly

[AngheloAlf]

Fixes #391

This is still a Work-In-Progress.

I'm opening this PR in the current state since I'm looking for people to actually test that this new feature is working properly.

To test this PR:

• Install / checkout this PR.

  python3 -m pip uninstall splat64
  python3 -m pip install git+https://github.com/AngheloAlf/splat.git@gpRelHack

• Set use_gp_rel_macro_nonmatching: False on your yaml
• Disable your own custom %gp_rel patching system.

If everything works as it should then running splat and then building your project should Just Work :tm:. Please let me know if it doesn't.

[AngheloAlf]

undrafted the PR

[ethteck]

looks grood, just need changelog