* decl.c (compatible_signatures_p): New function. The intended purpose
	is to check if two function signatures for a call-site and a callee are
	compatible enough for the call to be valid. The underlying purpose is
	to check if a call to a mapped builtin is using the right interface.
	The current code actually does not check antyhing - this a placeholder
	for future refinements.
	(gnat_to_gnu_entity) <E_Subprogram_Call>: Add preliminary bits to handle
	builtin calls for convention Intrinsic.

	* gigi.h (builtin_decl_for): Declare (new function).

	* utils.c (gnat_install_builtins): Install the target specific builtins.
	(builtin_decl_for): New function, provide a dummy body for now.

From-SVN: r92833