bind/objc: remove a constructor and retain blocks

A framework generated with gomobile bind -target=ios has two global
constructors: one initializing a data structure and another using it.
These constructors are defined in different translation units, which
(I believe, reasoning from C++ global constructors) means their order
of initialization is undefined.

A capturing block is stack allocated. Its memory is invalid after the
function returns. Make a copy of the interface initializer blocks so
they can be saved to the heap.

Block implementation background:
http://www.cocoawithlove.com/2009/10/how-blocks-are-implemented-and.html

Updates golang/go#12590

Change-Id: Ia7ae9f4bbd8df6e6e79949de54b3e6c48148c700
Reviewed-on: https://go-review.googlesource.com/14549
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/objc/seq_darwin.m

@@ -88,15 +88,17 @@
 // registered by calling go_seq_register_proxy from a global contructor funcion.
 static goSeqDictionary *proxies = NULL;
 
-__attribute__((constructor)) static void go_seq_very_init() {
-  proxies = [[goSeqDictionary alloc] init];
-}
-
 void go_seq_register_proxy(const char *descriptor,
                            void (*fn)(id, int, GoSeq *, GoSeq *)) {
-  [proxies put:^(id obj, int code, GoSeq *in, GoSeq *out) {
+  if (proxies == NULL) {
+    proxies = [[goSeqDictionary alloc] init];
+  }
+  // Copying moves the block to the heap.
+  id block = [^(id obj, int code, GoSeq *in, GoSeq *out) {
     fn(obj, code, in, out);
-  } withKey:[NSString stringWithUTF8String:descriptor]];
+  } copy];
+
+  [proxies put:block withKey:[NSString stringWithUTF8String:descriptor]];
 }
 
 // RefTracker encapsulates a map of objective-C objects passed to Go and
@@ -322,7 +324,10 @@ void go_seq_recv(int32_t refnum, const char *desc, int code, uint8_t *in_ptr,
 
   NSString *k = [NSString stringWithUTF8String:desc];
 
-  proxyFn fn = [proxies get:k];
+  proxyFn fn = NULL;
+  if (proxies != NULL) {
+    fn = [proxies get:k];
+  }
   if (fn == NULL) {
     LOG_FATAL(@"cannot find a proxy function for %s", desc);
     return;