Only compress debug sections if this saves space

[Cary Coutant]

> This patch has now remained unreviewed for a week.

Both H. J. and I suggested that you make the testcase large enough to
guarantee that all debug sections would be compressed.

If you're going to just discard all the new frags created by the trial
compression, I wonder if you should create those on a new obstack and
then discard that obstack before returning? (I suppose any memory leak
here would be minor, since the only time we'll choose not to use the
compressed frags is when they're quite small.)

-cary