Add plan9 support

Plan 9 has no ldd requirement/support.

Signed-off-by: Ronald G. Minnich <[email protected]>

ldd/ldd_plan9.go

@@ -0,0 +1,19 @@
+// Copyright 2024 the u-root Authors. All rights reserved
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ldd
+
+// List returns nothing.
+//
+// It's not an error for a file to not be an ELF.
+func List(names ...string) ([]string, error) {
+	return nil, nil
+}
+
+// FList returns nothing.
+//
+// It's not an error for a file to not be an ELF.
+func FList(names ...string) ([]string, error) {
+	return nil, nil
+}