replace stdatomic.h with zinnia/atomic.h

core/percpu.c

@@ -1,10 +1,10 @@
 #include <zinnia/alloc.h>
+#include <zinnia/atomic.h>
 #include <zinnia/cmdline.h>
 #include <zinnia/compiler.h>
 #include <zinnia/percpu.h>
 #include <zinnia/utils.h>
 #include <arch/percpu.h>
-#include <stdatomic.h>
 
 extern uint8_t __ld_percpu_start[];
 extern uint8_t __ld_percpu_end[];
@@ -37,7 +37,7 @@ struct percpu* percpu_new() {
 
     struct percpu* cpu = HHDM_PTR(mem);
     cpu->self = cpu;
-    cpu->id = atomic_fetch_add_explicit(&num_cpus, 1, memory_order_relaxed);
+    cpu->id = atomic_fetch_add(&num_cpus, 1, ATOMIC_RELAXED);
     cpu->online = false;
     return cpu;
 }