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/context.c

@@ -116,7 +116,7 @@ void arch_extracontext_save(extracontext_t *context) {
 	}
 }
 
-void arch_extracontext_load(extracontext_t *context) {
+KCOV_DISABLED void arch_extracontext_load(extracontext_t *context) {
 	if (current_thread()->proc && current_thread()->proc->ldt)
 		arch_gdt_set_ldt(current_thread()->proc->ldt, 0xffff);
 	else