This patch adds a new mechanism to have the Ada front end query what types are
supported by the back end. Types other than floating point types are currently
ignored. The information is needed because the list of floating point types
that may be present is open-ended, so it is impossible to add querying
functions for each type.

The main immediate use is for correct support of Interfaces.C.long_double,
when that type differs form Long_Long_Float. Types without definition in
the Ada Standard package must be explicitly defined using both precision
and range, and possibly representation attributes. Support for this will
be added in subsequent patches.

This patch also prepares for future addition of decimal floating point,
as well as potential native support of VMS floating point types.

Tested on x86_64-pc-linux-gnu, committed on trunk

2011-08-02  Geert Bosch  <bosch@adacore.com>

	* back_end.ads (Register_Type_Proc): New call back procedure type for
	allowing the back end to provide information about available types.
	(Register_Back_End_Types): New procedure to register back end types.
	* back_end.adb (Register_Back_End_Types): Call the back end to enumerate
	available types.
	* cstand.adb (Back_End_Float_Types): New list for floating point types
	supported by the back end.
	(Build_Float_Type): Add extra parameter for Float_Rep_Kind.
	(Copy_Float_Type): New procedure to make new copies of predefined types.
	(Register_Float_Type): New call back procedure to populate the BEFT list
	(Find_Back_End_Float_Type): New procedure to find a BEFT by name
	(Create_Back_End_Float_Types): New procedure to populate the BEFT list.
	(Create_Float_Types): New procedure to create entities for floating
	point types predefined in Standard, and put these and any remaining
	BEFTs on the Predefined_Float_Types list.
	* stand.ads (Predefined_Float_Types): New list for predefined floating
	point types that do not have declarations in package Standard.