Well, ok :) but according to the manual, I should do :

cd /sys/src/cmd/cc; mk install
# choose the appropriate compiler for your architecture
cd /sys/src/cmd/6c; mk install

which works for cc, but doesn't for 6c.

I guess I don't understand how to rebuild assembler, compiler and linker :/

I know it's not the right place to ask, but I'm learning and really appreciate your help.