VMA section overlap warnings for overlays

[David Stubbs]

> Committed.  I didn't see any occurrence of the problem Jan
> originally
> reported in http://sourceware.org/ml/binutils/2008-
> 05/msg00235.html on
> i386 or powerpc using recent compilers so perhaps some other
> change
> cured it.

Thanks Alan. I've tested this and it fixes the problems we were having.
I've modified the warning slightly though (in our sources) to only emit
it if the section VMA != LMA - as in there's a chance that this is an
overlay. 

Dave.