[Ada] Adjust traceback data stored for the pc symbolizer on alpha-vms

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

The VMS service we use to symbolize a callstack traceback entry is
advertized as expecting a PC and a frame pointer to identify an
invocation and we were honoring this.

Expectedly, this only works for live invocations, which represents
a fairly severe restriction on the possible uses of this service.

We are switching back to providing a procedure value instead of a frame
pointer to circumvent this limitation, and will rely on a vms service
accepting that instead.

2007-12-19  Olivier Hainque  <hainque@adacore.com>

	* tb-alvms.c (struct tb_entry_t, __gnat_backtrace): Revert back to use
	of Procedure Value instead of Frame Pointer as the invocation
	identifier associated with the instruction pointer in each traceback
	entry.

	* g-trasym-vms-alpha.adb (Traceback_Entry, PV_For, FP_For,
	TB_Entry_For): Revert back to use of Procedure Value instead of Frame
	Pointer as the invocation identifier passed to tbk$symbolize.

	* s-traent-vms.ads, s-traent-vms.adb
	(Traceback_Entry, PV_For, FP_For, TB_Entry_For): Revert back to use of
	Procedure Value instead of Frame Pointer as the invocation identifier
	passed to tbk$symbolize.

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