Add Go build support for GN.

mojo/go/system/impl/core_impl.go

+package impl;
+
+/*
+#include "mojo/public/platform/native/system_thunks.h"
+*/
+import "C"
+import "mojo/public/go/mojo/system"
+
+
+type CoreImpl struct {
+}
+
+func (c CoreImpl) GetTimeTicksNow() int64 {
+        return (int64)(C.MojoGetTimeTicksNow());
+}
+
+var lazyInstance *CoreImpl = nil;
+
+func GetCore() system.Core {
+        if lazyInstance == nil {
+                lazyInstance = new(CoreImpl);
+        }
+        return lazyInstance;
+}