Call enable_interrupts when initialising IRQs

Fixes raspberrypi/pico-examples#584, supercedes raspberrypi/pico-examples#588

Author: will-v-pi

src/rp2_common/hardware_irq/irq.c

@@ -666,6 +666,8 @@ void irq_add_tail_to_free_list(struct irq_handler_chain_slot *slot) {
 #endif
 
 __weak void runtime_init_per_core_irq_priorities(void) {
+    // enable interrupts that might be disabled by a previous bootloader stage
+    enable_interrupts();
 #if PICO_DEFAULT_IRQ_PRIORITY != 0
 #ifndef __riscv
     // static_assert(!(NUM_IRQS & 3), ""); // this isn't really required - the reg is still 32 bit