split irq.yaml output in nvic.h and vector_nvic.h

the weak pragmas need to be used in the very compilation unit where
their target is defined, requiring another dispatch

lib/cm3/vector.c

@@ -22,6 +22,8 @@
 
 /* load optional platform dependent initialization routines */
 #include "../dispatch/vector.c"
+/* load the weak symbols for IRQ_HANDLERS */
+#include <libopencm3/dispatch/vector_nvic.h>
 
 #define WEAK __attribute__ ((weak))