kcov: initial implementation

Implement a device (/dev/kcov) that exposes coverage info for the
kernel. This will be especially useful for coverage-based fuzzing (e.g.
syzkaller).

Exclude a lot of functions which would crash/otherwise interfere in the
coverage, including only processing coverage callbacks when not handling
an irq.

kernel-src/sys/thread.c

@@ -57,6 +57,10 @@ thread_t *sched_newthread(void *ip, size_t kstacksize, int nice, proc_t *proc, v
 }
 
 void sched_destroythread(thread_t *thread) {
+#ifdef KCOV_ENABLED
+	if (thread->kcov)
+		kcov_release(thread->kcov);
+#endif
 	mm_unmap(thread->kernelstack, thread->kernelstacksize, 0);
 	arch_extracontext_free(&thread->extracontext);
 	slab_free(thread_cache, thread);
@@ -101,6 +105,11 @@ __attribute__((noreturn)) void sched_threadexit() {
 		PROC_RELEASE(proc);
 	}
 
+#ifdef KCOV_ENABLED
+	if (current_thread()->kcov)
+		kcov_set_mode(KCOV_MODE_DISABLED);
+#endif
+
 	interrupt_set(false);
 	arch_context_saveandcall(threadexit_internal, current_cpu()->schedulerstack, NULL);
 	__builtin_unreachable();