files.c (find_file_in_dir): Always try to shorten for DOS non-system headers.

2014-05-09  Joey Ye  <joey.ye@arm.com>

	* files.c (find_file_in_dir): Always try to shorten for DOS
	non-system headers.
	* init.c (ENABLE_CANONICAL_SYSTEM_HEADERS): Default enabled for DOS.

From-SVN: r210264