ada: Make the names of uninstalled cross-gnattools consistent across builds

We suffer from an inconsistency in the names of uninstalled gnattools
executables in cross-compiler configurations.  The cause is a recipe we
have:

ada.all.cross:
	for tool in $(ADA_TOOLS) ; do \
	  if [ -f $$tool$(exeext) ] ; \
	  then \
	    $(MV) $$tool$(exeext) $$tool-cross$(exeext); \
	  fi; \
	done

the intent of which is to give the names of gnattools executables the
'-cross' suffix, consistently with the compiler drivers: 'gcc-cross',
'g++-cross', etc.

A problem with the recipe is that this 'make' target is called too early
in the build process, before gnattools have been made.  Consequently no
renames happen and owing to that they are conditional on the presence of
the individual executables the recipe succeeds doing nothing.

However if a target is requested later on such as 'make pdf' that does
not cause gnattools executables to be rebuilt, then 'ada.all.cross' does
succeed in renaming the executables already present in the build tree.
Then if the 'gnat' testsuite is run later on which expects non-suffixed
'gnatmake' executable, it does not find the 'gnatmake-cross' executable
in the build tree and may either catastrophically fail or incorrectly
use a system-installed copy of 'gnatmake'.

Of course if a target is requested such as `make all' that does cause
gnattools executables to be rebuilt, then both suffixed and non-suffixed
uninstalled executables result.

Fix the problem by moving the renaming of gnattools to a separate 'make'
recipe, pasted into a new 'gnattools-cross-mv' target and the existing
legacy 'cross-gnattools' target.  Then invoke the new target explicitly
from the 'gnattools-cross' recipe in gnattools/.

Update the test harness accordingly, so that suffixed gnattools are used
in cross-compilation testsuite runs.

	gcc/ada/
	* gcc-interface/Make-lang.in (ada.all.cross): Move recipe to...
	(GNATTOOLS_CROSS_MV): ... this new variable.
	(cross-gnattools): Paste it here.
	(gnattools-cross-mv): New target.

	gnattools/
	* Makefile.in (gnattools-cross): Also build 'gnattools-cross-mv'
	in GCC_DIR.

	gcc/testsuite/
	* lib/gnat.exp (local_find_gnatmake, find_gnatclean): Use
	'-cross' suffix where testing a cross-compiler.