Add Go build support for GN.

mojo/go/system/impl/core_impl.go

Index: mojo/go/system/impl/core_impl.go
diff --git a/mojo/go/system/impl/core_impl.go b/mojo/go/system/impl/core_impl.go
new file mode 100644
index 0000000000000000000000000000000000000000..5c34b2ee747d82b6ffbb4d50ceb0c1c2d1309df0
--- /dev/null
+++ b/mojo/go/system/impl/core_impl.go
@@ -0,0 +1,24 @@
+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;
+}