build: Use compile_time_request system for init, tasks, and shutdown

Avoid using linker magic to define the init, task, and shutdown
functions.  Instead, use the compile_time_request system.  This
simplifies the build and produces more efficient code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/armcm_irq.c

@@ -40,7 +40,7 @@ irq_poll(void)
 }
 
 // Clear the active irq if a shutdown happened in an irq handler
-static void
+void
 clear_active_irq(void)
 {
     uint32_t psr;

src/generic/timer_irq.c

@@ -82,7 +82,7 @@ timer_dispatch_many(void)
 
 // Periodic background task that temporarily boosts priority of
 // timers.  This helps prioritize timers when tasks are idling.
-static void
+void
 timer_task(void)
 {
     irq_disable();
@@ -91,7 +91,7 @@ timer_task(void)
 }
 DECL_TASK(timer_task);
 
-static void
+void
 timer_irq_shutdown(void)
 {
     timer_repeat_until = timer_read_time() + TIMER_IDLE_REPEAT_TICKS;