kernel: add a helper to retrieve the bsp CPU

This is useful to be able to e.g. set affinity for the BSP CPU.

kernel-src/arch/x86-64/main.c

@@ -124,3 +124,7 @@ void kernel_entry() {
 	sched_runinit();
 	sched_threadexit();
 }
+
+cpu_t *_bsp(void) {
+	return &bsp_cpu;
+}