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/arch/x86-64/cpu.c

@@ -140,7 +140,7 @@ bool arch_cpu_smap_enabled(void) {
 	return has_smap;
 }
 
-void arch_nmi_isr(context_t *ctx) {
+KCOV_DISABLED void arch_nmi_isr(context_t *ctx) {
 	// we are in an NMI context, running on IST0 
 	// first load the proper gsbase to get access to cpu-local variables. 
 	// the pointer to cpu's data is at the beggining of the stack so after the context struct.