runtime,os: add os.Executable() for Darwin

src/os/executable_darwin.go

@@ -0,0 +1,10 @@
+//go:build darwin
+
+package os
+
+// via runtime because we need argc/argv ptrs
+func runtime_executable_path() string
+
+func Executable() (string, error) {
+	return runtime_executable_path(), nil
+}