PR 34204 dlltool SEGVs with --exclude-symbols

This fixes a segfault introduced by commit 45a7f5a29de7 which didn't
take into account that there was a use of leading_underscore in option
processing.

	PR 34204
	* dlltool.c (struct string_list): Make string a flexible array
	member.
	(add_excludes): Adjust to suit.  Extract code adding underscore
	and informing..
	(underscore_excludes): ..to here.  New function.
	(main): Call underscore_excludes.

(cherry picked from commit 500390720267e2f009f0354e5051fa2e176b143d)