[gdb/build] Cleanup gdb/features/feature_to_c.sh

Clean up script gdb/features/feature_to_c.sh by:
- fixing shellcheck warnings,
- moving an embedded awk script out of the file, reducing the amount of
  escaping in the awk script, making it more readable and maintainable, and
- adding emacs / vi settings for local tab size 2 (copied from ./ltmain.sh).

Tested on x86_64-linux.

Approved-by: Kevin Buettner <kevinb@redhat.com>