Fair enough. I guess you're talking about microcontrollers rather than lightweight CPUs, at least in my experience it's still standard to use GCC on the sort of dinky ARM-based devices my company produces.

I'll note that GPL never says anything about upstreaming. It can certainly make life easier if you want to keep your patches up to date and hand off maintenance responsibility, but it suffices to make a written offer of source code under the same license (on a conventional medium) to your customers to comply with any GPL version. You're never obliged to upstream or even publish your code.