Add kcov(4), a kernel code coverage tracing driver. It's used in conjunction

Add kcov(4), a kernel code coverage tracing driver. It's used in conjunction
with the syzkaller kernel fuzzer. So far, 8 distinct panics have been found and
fixed. This effort will continue.

kcov is limited to architectures using Clang as their default compiler and is
not enabled by default.

With help from mpi@, thanks!

ok kettenis@ mpi@ visa@