Ability to build the GNAT runtime with project files

Ability to build the GNAT runtime with project files

This change add project files to provide the ability to rebuild the
runtime with gprbuild after setup-rts is called.

gcc/ada/

	* Makefile.rtl (ADA_INCLUDE_SRCS): Replace Makefile.adalib by
	libada.gpr and associated project files.
	(g-debpoo.o): Add missing rule to ensure subprograms are not reordered.
	(setup-rts): Add generation of libgnat/libgnarl.lst.
	(LIBGNAT_SRCS): Remove thread.c which is part of libgnarl.
	* tracebak.c, tb-gcc.c: Merged the two files to simplify dependencies.
	* libgnarl/libgnarl.gpr, libgnat/libada.gpr,
	libgnat/libgnat.gpr, libgnat/libgnat_common.gpr: New files.
	* doc/gnat_ugn/the_gnat_compilation_model.rst: Makefile.adalib
	replaced by libada.gpr.
	* libgnat/system-mingw.ads: Remove obsolete comment.
	* gcc-interface/Makefile.in: Remove dependency on tb-gcc.c.