Make dg-extract-results.sh explicitly treat .{sum,log} files as text

Make dg-extract-results.sh explicitly treat .{sum,log} files as text

This merges Sergio's fix from GCC:
  https://gcc.gnu.org/ml/gcc-patches/2014-12/msg01293.html

gdb/testsuite/ChangeLog:
2015-03-09  Pedro Alves  <palves@redhat.com>

	Merge dg-extract-results.sh from GCC upstream (r218843).

	2014-12-17  Sergio Durigan Junior  <sergiodj@redhat.com>
		* dg-extract-results.sh: Use --text with grep to avoid issues with
		binary files.  Fall back to cat -v, if that doesn't work.