[ROSBE-Unix] Install amd64 gcc toolchain along with i386 one

[zefklop]

Voilà

[ColinFinck]

Hey Jérôme. I've integrated your changes in #15.
However, I chose to have a separate script for building the amd64 compiler. Building RosBE already takes long enough and there may be people, who have to build from source but only need the i386 compiler.

The design of RosBE-Unix has long been prepared for additional compiler add-ons like amd64, and now we can finally make use of that :)