Reduce absolute paths in generated ninja files

tools/generate_library_loader/generate_library_loader.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This template makes a stub for a Linux system library that dynamically loads
 # it at runtime.
 
 # name: Name to use for the value of the --name arg.
 # output_h/output_cc: Names for the generated header/cc file with no dir.
 # header: header file to process. Example: "<foo/bar.h>"
@@ skipping 12 matching lines @@
 
     outputs = [
       output_h,
       output_cc,
     ]
 
     args = [
       "--name",
       invoker.name,
       "--output-h",
-      rebase_path(output_h),
+      rebase_path(output_h, root_build_dir),
       "--output-cc",
-      rebase_path(output_cc),
+      rebase_path(output_cc, root_build_dir),
       "--header",
       invoker.header,
 
       # Note GYP build exposes a per-target variable to control this, which, if
       # manually set to true, will disable dlopen(). Its not clear this is
       # needed, so here we just leave off. If this can be done globally, we can
       # expose one switch for this value, otherwise we need to add a template
       # param for this.
       "--link-directly=0",
     ]
@@ skipping 12 matching lines @@
     }
     sources = [
       output_cc,
       output_h,
     ]
     public_deps = [
       ":${target_name}_loader",
     ]
   }
 }