ARM cross compiler GCC 6 for Node >= 12

[rvagg]

I now have GCC 6.5.0 and GCC 8.3.0 @ https://github.com/rvagg/rpi-newer-crosstools, which is already cloned to /opt/raspberrypi/rpi-newer-crosstools/ on the cross compiler machines (it needs an update). This update allows workers with new labels like "cross-compiler-armv7-gcc-6" to select GCC 6 for ARMv7. It still allows ARMv6 even though we're dropping that for Node 12, and we also don't need GCC 8 support yet but we get some flexibility here for experimentation.

I'd appreciate some sanity checking on the VersionSelectorScript before I merge this to try and avoid the problems with centos7 that I caused yesterday. Note the labels we currently use: https://ci.nodejs.org/view/All/job/node-cross-compile/, we'll also add "cross-compiler-armv7-gcc-6" to that list.

[rvagg]

Closes #1731

[rvagg]

I'm going to bite the bullet, cross my fingers and put this into action, will have to do some testing across both ci and ci-release with different branches.

[rvagg]

Had to update with #1782 to get the host gcc-6 compiler onto the machines.

Confirmed working:

• master test uses gcc-6: https://ci.nodejs.org/job/node-test-commit-arm-fanned/8124/
• 10.x test uses gcc-4.9.4: https://ci.nodejs.org/job/node-test-commit-arm-fanned/8125/
• 8.x test uses gcc-4.8: https://ci.nodejs.org/job/node-test-commit-arm-fanned/8126/
• master release uses gcc-6: https://ci-release.nodejs.org/job/iojs+release/4390/
• 10.x release uses gcc-4.9.4: https://ci-release.nodejs.org/job/iojs+release/4391/
• 8.x release uses pi1-docker: https://ci-release.nodejs.org/job/iojs+release/4392/

@refack I've changed pi2-docker to 'stretch' for node-test-binary-arm-12+, https://ci.nodejs.org/job/node-test-binary-arm-12+/108/ (#1783 is similar but for the scaleway armv7 machines).