Ada: Replace hardcoded GNAT commands for GNAT tools

Ada: Replace hardcoded GNAT commands for GNAT tools

This replaces the hardcoded gnat{make,link,bind,ls} commands with expansion
of the GNAT{MAKE,BIND} variables computed by the configure machinery, during
the build of the GNAT tools.

The default GNATMAKE_FOR_HOST duplicates the default GNATMAKE, and someone
setting GNATMAKE in the toplevel configuration may want it applied for all
host compilations.  Direct assignment of GNATMAKE_FOR_HOST keeps working.

gcc/ada/
	PR ada/120106
	* gcc-interface/Make-lang.in: Set GNAT{MAKE,BIND,LINK_LS}_FOR_HOST
	from GNAT{MAKE,BIND} instead of using hardcoded commands.
gnattools/
	PR ada/120106
	* configure.ac: Remove ACX_NONCANONICAL_HOST and add ACX_PROG_GNAT.
	* configure: Regenerate.
	* Makefile.in: Do not substitute host_noncanonical but substitute
	GNATMAKE and GNATBIND.
	Set GNAT{MAKE,BIND,LINK_LS}_FOR_HOST from GNAT{MAKE,BIND} instead
	of using hardcoded commands.