PR24806, Linking with -T inside --start-group/--end-group

PR24806, Linking with -T inside --start-group/--end-group

This patch processes INSERT AFTER and INSERT BEFORE in a user -T
script when such a script is invoked on the command line inside
--start-group/--end-group.  Also, ld now warns when the user simply
forgot --end-group.

	PR 24806
	* ldlang.c (process_insert_statements): Add start of list
	parameter.  Use rather than lang_os_list.head.  Process insert
	statements inside group statements with a recursive call.
	(lang_process): Adjust process_insert_statements call.
	* lexsup.c (parse_args): Warn when adding missing --end-group.