Enable the use of pwrite in the object writers.

[Rafael Espíndola]

> * 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

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.

Cheers,
Rafael
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