Since the combined size of the gcc shared libs in /usr/local/lib add to only 225kb, my vote would be to have them in a gcc-libs extension to be used by apps during runtime and to be a dep of course of gcc.
That way when gcc is updated, we don't have to hunt down each extension that contains gcc lib files and update the libs in them when gcc is updated. If those files are left in the extensions after the next gcc update, they would cause conflict with the newer gcc. The size increase of extensions packing gcc libs in them is less of a concern than the conflict that would later occur after gcc update, but the less size would be a bonus.
This would make an easier transition whenever the toolchain is updated.