[Ada] Add support for 64bit descriptors on VMS

[Arnaud Charlet <charlet at adacore dot com>]

A recent enhancement to the GNAT runtime on VMS enables use of the
64bit heap.  Passing pointers allocated on the 64bit heap via descriptor
requires the use of a 64bit descriptor. This change implements that fix.

Manually tested on VMS with GCC 4.1, Tested on i686-pc-linux-gnu
Committed on trunk.

2008-07-30  Doug Rupp  <rupp@adacore.com>

	* gigi.h (build_vms_descriptor64): New function prototype.
	(fill_vms_descriptor): Modified function prototype.

	* utils.c (build_vms_descriptor64): New function.

	* utils2.c (fill_vms_descriptor): Fix handling on 32bit systems.

	* trans.c (call_to_gnu): Call fill_vms_descriptor with new third
	argument.

	* decl.c (gnat_to_gnu_tree): For By_Descriptor mech, build both a
	64bit and 32bit descriptor and save the 64bit version as an alternate
	TREE_TYPE in the parameter.
	(make_type_from_size) <RECORD_TYPE>: Use the appropriate mode for the
	thin pointer.

	* ada-tree.h (DECL_PARM_ALT, SET_DECL_PARM_ALT): New macros.

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