stdio_usb improvements (#871)

Use shared IRQ if available to avoid 1ms timer. Allow use of stdio_usb with user's tinyusb setup if it has CDC

src/rp2_common/hardware_irq/include/hardware/irq.h

@@ -252,6 +252,14 @@ void irq_add_shared_handler(uint num, irq_handler_t handler, uint8_t order_prior
  */
 void irq_remove_handler(uint num, irq_handler_t handler);
 
+/*! \brief Determine if the current handler for the given number is shared
+ *  \ingroup hardware_irq
+ *
+ * \param num Interrupt number \ref interrupt_nums
+ * \param return true if the specified IRQ has a shared handler
+ */
+bool irq_has_shared_handler(uint num);
+
 /*! \brief Get the current IRQ handler for the specified IRQ from the currently installed hardware vector table (VTOR)
  * of the execution core
  *  \ingroup hardware_irq

src/rp2_common/hardware_irq/irq.c

@@ -95,10 +95,21 @@ static int8_t irq_hander_chain_free_slot_head;
 static inline bool is_shared_irq_raw_handler(irq_handler_t raw_handler) {
     return (uintptr_t)raw_handler - (uintptr_t)irq_handler_chain_slots < sizeof(irq_handler_chain_slots);
 }
+
+bool irq_has_shared_handler(uint irq_num) {
+    check_irq_param(irq_num);
+    irq_handler_t handler = irq_get_vtable_handler(irq_num);
+    return handler && is_shared_irq_raw_handler(handler);
+}
+
 #else
 #define is_shared_irq_raw_handler(h) false
+bool irq_has_shared_handler(uint irq_num) {
+    return false;
+}
 #endif
 
+
 irq_handler_t irq_get_vtable_handler(uint num) {
     check_irq_param(num);
     return get_vtable()[16 + num];