Fix Normalize_Pathname to avoid a constraint error.

2018-05-29  Pascal Obry  <obry@adacore.com>

gcc/ada/

	* libgnat/s-os_lib.adb (Normalize_Pathname): Fix handling of ".." in
	the root directory.

gcc/testsuite/

	* gnat.dg/normalize_pathname.adb: New testcase.

From-SVN: r260872