add ASM exam environment

What?

Declare a gcc-as package that removes the gcc binary while keeping everything else (as, g++, glibc...)

Why?

The prevent any cheating using C code for the assembly exam. We keep the g++ binary for the provided tests suite.

How?

By creating a wrapper around the gcc package to delete the gcc binary.

Note to reviewer

• clang patched package is missing.
• Building target .#gcc-as is working.
• Building a exam-asm-docker image is currently failing (help welcomed!)