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;
+}

kernel-src/include/x86-64/arch/cpu.h

@@ -54,6 +54,8 @@ static inline uint16_t be_to_cpu_w(uint16_t w) {
 	return __builtin_bswap16(w);
 }
 
+cpu_t *_bsp(void);
+
 static inline cpu_t *_cpu() {
 	cpu_t *cpu;
 	asm volatile ("mov %%gs:0, %%rax" : "=a"(cpu) : : "memory");