[stm32] add better way of handling irq vectors

-define a default dummy vector and alias all of the available irq handlers
to this vector. Using a standard naming convention, you can override the
handler from anywhere without modifying the vector table.

target/stm32-p107/include/target/debugconfig.h

@@ -27,5 +27,6 @@
 
 #define DEBUG_UART USART3
 #define DEBUG_UART_IRQ USART3_IRQn
+#define DEBUG_UART_IRQ_HANDLER stm32_USART3_IRQ
 
 #endif