pw_toolchain: Include cross reference table in map files

Change-Id: Icf034a5cf9ee0cf77edd0cf5a5f1cf1d841e4703
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/151051
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Alex Brachet <abrachet@google.com>
Reviewed-by: Petr Hosek <phosek@google.com>
Reviewed-by: Keir Mierle <keir@google.com>

pw_toolchain/generate_toolchain.gni

@@ -269,6 +269,7 @@ template("generate_toolchain") {
         _link_flags += [
           # Output a map file that shows symbols and their location.
           "-Wl,-Map,$_link_mapfile",
+          "-Wl,--cref",
         ]
       }