Emit config when building r8lib

This is useful for debugging the r8lib build.

Change-Id: If526cbc0e7078da0748893fa6dd8b8ca1d933808
diff --git a/tools/create_r8lib.py b/tools/create_r8lib.py
index e8bf4f2..c8eb00e 100755
--- a/tools/create_r8lib.py
+++ b/tools/create_r8lib.py
@@ -103,6 +103,7 @@
     cmd.extend(['--map-id-template', map_id_template])
     cmd.extend(['--source-file-template', source_file_template])
     cmd.extend(['--output', args.output])
+    cmd.extend(['--pg-conf-output', args.output + '.config'])
     cmd.extend(['--pg-map-output', args.output + '.map'])
     cmd.extend(['--partition-map-output', args.output + '_map.zip'])
     cmd.extend(['--lib', jdk.GetJdkHome()])