cpu: cortexm_common: add define for number of non-isr exceptions

cpu/cortexm_common/include/vectors_cortexm.h

@@ -34,6 +34,14 @@ extern "C" {
  */
 #define ISR_VECTORS     __attribute__((used,section(".vectors")))
 
+/**
+ * @brief   Number of Cortex-M non-ISR exceptions
+ *
+ * This means those that are no hardware interrupts, or the ones with a
+ * negative interrupt number.
+ */
+#define CPU_NONISR_EXCEPTIONS   (15)
+
 /**
  * @brief   This function is the default entry point after a system reset
  *