[gitlab CI] Build stdlib using sac2c package

Currently, as a part of CI pipeline, we always build stdlib using sac2c binaries directly from the working build directory. The problem with this is that we never test whether resulting package is operational or not. It would be a better idea to extend the pipeline as follows:

1. Compile sac2c
2. Create sac2c package
3. Install sac2c package & build stdlib

This would allow to identify the cases when packages are not fully operational.