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/main.c

@@ -20,7 +20,7 @@ DEFINE_KERNEL_ARGUMENT(rootfs, char *);
 DEFINE_KERNEL_ARGUMENT(initrd, bool);
 DEFINE_KERNEL_ARGUMENT(root_wait, bool);
 
-void kernel_entry() {
+KCOV_DISABLED void kernel_entry() {
 	cpu_set(&bsp_cpu);
 
 #ifdef TERM_EARLY_INIT