Node.js 12 TODO: GCC 6 toolchain for ARMv7

[rvagg]

I have a 6.3.1 @ https://github.com/rvagg/rpi-newer-crosstools and it's on the cross-compiler machine, it's just not in use. We currently switch between 4.8 and 4.9 (from that repo) depending on which Node version, we need an additional switch to 6 but we also need confirmation that it even works!

>= Debian 9 only for Node 12.

[github-actions]

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