[Ada] Fix traceback computation on ppc

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

Some targets, specifically the PPC-based ones, arrange for a dummy
function to be called at the very beginning of the computation. This
makes it easier to retrieve the return address saved by the function.
This was disabled because the call was optimized away by the compiler.

2006-02-13  Eric Botcazou  <ebotcazou@adacore.com>

	* tracebak.c (PPC AIX/Darwin): Define FORCE_CALL to 1.
	(PPC VxWorks): Likewise.
	(Generic unwinder): Define FORCE_CALL to 0 if not already defined.
	(forced_callee): Make non-inlinable and non-pure.
	(__gnat_backtrace): Call forced_callee if FORCE_CALL is set to 1.

Attachment: difs.84
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