m68k support

(Cross linking from https://github.com/mattgodbolt/compiler-explorer/issues/271)

Hi,

So after spending a bit of time fixing up my build script for m68k compilers I thought I'd take a look and see how it could be added to this repository. I have a few initial questions:

1. From what I've seen from the other cross compiler build scripts (like for example gcc-cross\build\msp430-gcc453\) only the compiler is being built without libraries. My script also builds quite a few libs - are those not needed? I'm asking this because if only the barebones compilers are required then the build script gets significantly smaller.
1. I also asked this in the compiler-explorer issue - from what I understand the compilers are being installed in /opt - what would an accepted naming scheme be? @RabsRincon suggested ```gcc-x-x-x-68k```, by default the compilers are named ```m68k-x-x-x-elf``` fwiw.
1. Can anyone help me figuring out how docker works? For my build I require a few versions of gcc to be present (4 through 7), the usual GMP/MPFR/MPC and a couple of others like flex and bison (if the libraries are being built). From what I understand there's a repository of docker packages one can get and use the built-in tools but this is just dark arts for me right now :)

Feedback welcome!
George