rp2040: add multicore support

src/internal/task/task_stack_cortexm.c

@@ -1,4 +1,4 @@
-//go:build scheduler.tasks && cortexm
+//go:build (scheduler.tasks || scheduler.cores) && cortexm
 #include <stdint.h>
 
 uintptr_t SystemStack() {

src/internal/task/task_stack_cortexm.go

@@ -1,4 +1,4 @@
-//go:build scheduler.tasks && cortexm
+//go:build (scheduler.tasks || scheduler.cores) && cortexm
 
 package task