Update the default version of GCC pulled in via USE_GCC=yes and a myriad
other ways from GCC 7 (7.4 right now) to GCC 8 (8.2 right now).

This is the first time for eons, if not forever, that we are on the most
recent major version of GCC, a catch-up that's been taking more than two
years with the great help of many fellow committers and volunteers.

PR: 231590
Tested by: antoine (exp-runs)
Thanks to: jbeich, tobik, jwb, mi, yuri, and others for helping fix (broken) ports