Upgrade gcc toolchain version

This core still uses the rather old gcc 4.8 toolchain, which lacks a lot of useful (C++) functions.

I just tried replacing it with SAMD's gcc 7 version, which seems to work just fine with a few test sketches I tried. Maybe this would be an easy improvement: Just replace the gcc version with the one from SAMD.

Locate the core's gcc 4.8 toolchain configuration and compare it with the SAMD gcc 7 toolchain mentioned in the issue. Build the core and run the few test sketches referenced; done means the upgraded toolchain works without breaking those sketches.

Written by the indexing model from the issue text.