[Ada] use system unwinder for computing backtrace under IRIX

[Arnaud Charlet <charlet at adacore dot com>]

[Ada] use system unwinder for computing backtrace under IRIX

Note that this and the following set of changes have been tested by
bootstrapping and testing at -O0, to work around unrelated regressions
that prevent proper build of the Ada compiler.

Manually tested under mips-irix and sparc-solaris
Tested on i686-linux, committed on mainline.

After corresponding changes in the mips back-end, it is now possible
to use the system unwinder (libexc) for computing tracebacks under IRIX.

Also, a solaris v9 clean up:
The V9 ABI is that of the 64-bit mode so, in this mode, the value of the
stack pointer %sp or the frame pointer %fp must never be used directly
(it is not even aligned on a word boundary!), but always after being
shifted by the stack bias, that is to say 2047.

2005-06-14  Olivier Hainque  <hainque@adacore.com>
	    Eric Botcazou  <ebotcazou@adacore.com>

	* s-mastop-irix.adb (Pop_Frame): Revert shorcuts avoiding calls to
	exc_unwind, now that we are generating proper .debug_frame output for
	that target.

	* tracebak.c: Remove the mips-irix section, as we are now using the
	s-mastop based unwinder again.
	Under SPARC/Solaris, take into account the stack bias to compute the
	frame offset.  The stack bias is 0 for the V8 ABI and 2047 for the V9
	ABI.

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