Instruct macos users to use avr-gcc@7 (#3337)

If you run `brew install avr-gcc`, you get a version that has
compatibility issues with LUFA. I updated the getting started guide for
osx, the qmk_install setup script, and added a section to the FAQ for
folks like me who accidentally updated avr-gcc past 7.

util/macos_install.sh

@@ -22,4 +22,4 @@ fi
 brew tap osx-cross/avr
 brew tap PX4/homebrew-px4
 brew update
-brew install avr-gcc gcc-arm-none-eabi dfu-programmer avrdude
+brew install avr-gcc@7 gcc-arm-none-eabi dfu-programmer avrdude