[Ada] General purpose doubly linked list for compiler and tool use

[Ada] General purpose doubly linked list for compiler and tool use

This patch adds unit GNAT.Lists which currently contains the
implementation of a general purpose doubly linked list intended for use
by the compiler and the tools around it.

2018-08-21  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* impunit.adb: Add g-lists to the set of non-implementation
	units.
	* libgnat/g-lists.adb, libgnat/g-lists.ads: New unit.
	* Makefile.rtl: Add g-lists to the set of non-tasking units.
	* gcc-interface/Make-lang.in: Add g-lists to the set of files
	used by gnat1.

gcc/testsuite/

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

From-SVN: r263714