BUILD: pollers: use an initcall to register the pollers

Pollers are among the few remaining blocks still using constructors
to register themselves. That's not needed anymore since the initcalls
so better turn to initcalls.
diff --git a/src/ev_evports.c b/src/ev_evports.c
index c3be00e..4d61154 100644
--- a/src/ev_evports.c
+++ b/src/ev_evports.c
@@ -402,10 +402,8 @@
 }
 
 /*
- * This constructor must be called before main() to register the event ports
- * poller.
+ * Registers the poller.
  */
-__attribute__((constructor))
 static void _do_register(void)
 {
 	struct poller *p;
@@ -431,3 +429,5 @@
 	p->poll = _do_poll;
 	p->fork = _do_fork;
 }
+
+INITCALL0(STG_REGISTER, _do_register);