Include SPDX License Exceptions

[atrniv]

I was using askalono to scan the GCC codebase for licenses and found that it was unable to detect the COPYING.RUNTIME license which should be identified as GCC-exception-3.1. After some digging I found that this was because the askalono does not load the data from the json/exceptions folder in the SPDX dataset.

The changes required to load these additional license exceptions text is minimal, it would be great if this was added by default. I would be happy to raise a PR for the same if it makes sense.