[gdb/contrib] Add -c option to words.sh script

[gdb/contrib] Add -c option to words.sh script

The words.sh script in its current form extracts c comments from files, which
it then transforms into a list of words.

To use the script on the documentation (as I did for commit 6b92c0d3533
"[gdb/doc] Fix typos"), I needed to disable the "extract c comments" part.

Add an option -c that enables extracting c comments, and is off by default.

gdb/ChangeLog:

2019-11-25  Tom de Vries  <tdevries@suse.de>

	* contrib/words.sh: Add -c option.

Change-Id: Ifa34d435b3c41b3ff845dc07ae4b0d9f02d92a2d