epex should be enabled by default

src/core/lib/iomgr/ev_posix.c

@@ -62,11 +62,9 @@ typedef struct {
 } event_engine_factory;
 
 static const event_engine_factory g_factories[] = {
-    {"epoll1", grpc_init_epoll1_linux},
+    {"epollex", grpc_init_epollex_linux},   {"epoll1", grpc_init_epoll1_linux},
-    {"epollsig", grpc_init_epollsig_linux},
+    {"epollsig", grpc_init_epollsig_linux}, {"poll", grpc_init_poll_posix},
-    {"poll", grpc_init_poll_posix},
     {"poll-cv", grpc_init_poll_cv_posix},
-    {"epollex", grpc_init_epollex_linux},
 };
 
 static void add(const char *beg, const char *end, char ***ss, size_t *ns) {