Add gcc4 and gcc4-armhf packages

Requires pmbootstrap!1754 (merged)

I've followed @ollieparanoid steps described here to package them and succesfully tested both the gcc4 compiler on x86_64 with a simple program (fbdebug) and the gcc4-armhf cross-compiler to build a very old u-boot port for Galaxy Nexus (a MR will follow soon) which failed to boot with either newer version (6 and 8).