[Go] Make -intgosize optional

We no longer support Go < 1.2 so we can default intgo to ptrdiff_t
and uintgo to size_t.

Fixes #683
Fixes #2233

Lib/go/goruntime.swg

@@ -24,6 +24,7 @@ static void* Swig_malloc(int c) {
 %}
 
 %insert(cgo_comment_typedefs) %{
+#include <stddef.h>
 #include <stdint.h>
 %}