Fix code size reporting when input directory is missing a trailing '/'

run_smoke_bench script fails to report code size changes if you have a
trailing '/' in <old_build_dir> but not <new_build_dir>.

This change appends a separator if it is missing

Author: meg-gupta

benchmark/scripts/run_smoke_bench

@@ -272,6 +272,7 @@ def report_code_size(opt_level, old_dir, new_dir, platform, output_file):
     old_lines = ""
     new_lines = ""
     for oldfile in files:
+        new_dir = os.path.join(new_dir, '')
         newfile = oldfile.replace(old_dir, new_dir, 1)
         if os.path.isfile(newfile):
             oldsize = get_codesize(oldfile)