[Ada] add gcc-interface subdirectory

[Arnaud Charlet]

This change moves the interface between the Ada front-end and the GCC
middle-end to a subdirectory ada/gcc-interface, to help separating more
cleanly each parts of the compiler, and also to help AdaCore contribute
changes to mainline while using other back-end versions.

Tested manually by kludging trans.c which has been broken recently, and
tested on older revisions sucessfully, applied on trunk.

2008-07-29  Arnaud Charlet  <charlet@adacore.com>

	* gcc-interface: New directory.

	* ada-tree.def, cuintp.c, gigi.h, Makefile.in, targtyps.c, ada.h,
	utils.c, ada-tree.h, decl.c, lang.opt, Make-lang.in, trans.c,
	config-lang.in, deftarg.c, lang-specs.h, misc.c, utils2.c: Moved
	to gcc-interface subdirectory.