hack: document empty Goassembly file

Signed-off-by: Vicent Marti <vmg@strn.cat>

go/hack/runtime.s

@@ -0,0 +1,3 @@
+// DO NOT REMOVE: this empty goassembly file forces the Go compiler to perform
+// external linking on the sibling `runtime.go`, so that the symbols declared in that
+// file become properly resolved