[rfa/doco] PROBLEMS: add regressions since gdb 6.0

[Andrew Cagney]

>>I don't think it should retain regressions since 5.3, the file is 
>>> stating problems in this release, not the last one.
> 
> 
> These are still problems in this release.  They haven't been fixed.

There are problems in all the releases, we don't list them.

The objective here is to draw the users attention to places where gdb is 
both better and worse than the _last_ release (6.0).  Not a sorry list 
of problems dateing back to version 0.0.

Andrew