Enable the use of pwrite in the object writers.

[Justin Bogner]

Rafael Espíndola <rafael.espindola at gmail.com> writes:
>> * Have buffer_ostream contain a buffer. That is probably the best for
>> the users, but requires changing raw_svector_ostream to have a
>> protected init method since it would be constructed before the buffer.
>> I might still give this one a try.
>
> That was fairly easy. So patches with it are attached. I like it a bit better

These LGTM.

> Another possible variation that I forgot to mention on the original email:
>
> Have LLVMTargetMachine::addPassesToEmitFile create the buffer stream
> when seeking is not supported. The disadvantage is that it would
> require a "supportsSeeking()" method in the base, so it looks like a
> clumsy version of isa/dyn_cast. It would also not solve clang's other
> uses of streams.