bootstrap/project.conf prepare for per-arch optimizations

This MR is preliminary work for per-arch optimizations for bootstrap, similar to those in main project.conf. It's only code refactor, it shouldn't affect actual build.

I originally planned to crate MR with per-arch optimizations included but with current issues with CI it could take a very long time to test that everything works. I concluded that it's better to merge it in parts and this one is the biggest one in terms of visual changes in config thus landing it now will save time for rebasing in case of future code conflicts, i.e. changes in code linting.

Further changes will be very simple - they will just add specific buildflag to variables defined here.