Coverage-guided kernel fuzzing with syzkaller

> To start with, the compiler option to generate the needed coverage data has only recently been added to GCC
> (as -fsanitize-coverage=trace-pc), so the kernel needs to be built with a fresh-from-tip version of GCC.

the gcc-side instrumentation is very simple and could be done from a plugin, thus extending support all the way back to gcc 4.5.