cpu/cortexm_common: Make cpu.h IWYU clean

There is clearly no reason `cpu.h` should `#include <stdio.h>`.

Also add an export pragma to `cpu_conf.h`, as portable code is
expected to include `cpu.h` (which exists across MCU families in RIOT),
and not `cpu_conf.h` (which only exists for some MCU families).

cpu/cortexm_common/include/cpu.h

@@ -30,12 +30,10 @@
 #ifndef CPU_H
 #define CPU_H
 
-#include <stdio.h>
-
 #include "irq.h"
 #include "sched.h"
 #include "thread.h"
-#include "cpu_conf.h"
+#include "cpu_conf.h" /* IWYU pragma: export */
 
 #ifdef __cplusplus
 extern "C" {