Find lto plugin when using gcc-5.0

Find lto plugin when using gcc-5.0

I originally had this as --print-prog-name and changed back to
--print-file-name to suit older gcc, neglecting to check whether
gcc-5.0 --print-file-name finds the lto plugin.  It doesn't.

	* ld-plugin/lto.exp: Use both --print-file-name and --print-prog-name
	when looking for lto plugin.