kcov: Integrate with KMSAN

• kern_kcov.c needs to be compiled with -fsanitize=kernel-memory when KMSAN is configured since it calls into various other subsystems.
• Disable address and memory sanitizers in kcov(4)'s coverage sanitizer callbacks, as they do not provide useful checking. Moreover, with KMSAN we may otherwise get false positives since the caller (coverage sanitizer runtime) is not instrumented.
• Disable KASAN and KMSAN interceptors in subr_coverage.c, as they do not provide any benefit but do introduce overhead when fuzzing.

Sponsored by: The FreeBSD Foundation