[vta,vta4.3,trunk?] introduce -fcompare-debug

[Alexandre Oliva]

This patch introduces a better mechanism to test that the compiler
produces the same output with and without -g than the one I've been
using before.

With this option, the compiler will compile the same program twice,
dumping the final RTL and comparing the dumps.  This will enable us to
verify that -g and/or -fvar-tracking-assignments have no codegen
effects on our entire testsuite, and any other codebase you might want
to compile with -fcompare-debug.

This has already uncovered a number of codegen differences in a full
build, that had been missed by bootstrap-debug and
bootstrap-debug-lib.  I've already fixed some of them, but I still
have others to fix.

This also exposed a number of situations in which GCC compilers
(rather than drivers) dumped information to stdout or to files not
mentioned in the command line.  The patch below takes care of the ones
I've located so far.

One shortcoming of this approach is that it doesn't notice differences
that aren't represented as RTL, such as data variable definitions and
anything else that is not part of a function RTL.  I still haven't
tried to add variables, initializers and constant pools, but this
shouldn't be too hard, except that debug information output must be
prevented from causing differences in the dumps.

This could be useful for GCC even without VTA, to detect latent
errors.  Probably too late for 4.4, but hey, it doesn't hurt to ask.
Ok to install?

Patch about to be installed in vta follows.  After it, the small diff
between the patch for vta and the patch for vta4.3.

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-- 
Alexandre Oliva         http://www.lsd.ic.unicamp.br/~oliva/
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}
FSFLA Board Member       ?S? Libre! => http://www.fsfla.org/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}