Enable the use of pwrite in the object writers.

[Duncan P. N. Exon Smith]

> On 2015-Apr-10, at 15:15, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> 
>> Both this and the previous mail have callers of `supportsSeeking()` but no
>> definition (in llvm.patch).  Is the patch out-of-sync somehow?
>> 
> 
> No, that was a small addition in r234619 (to raw_fd_ostream only).

Ah, sorry; my tree was out-of-date.

FWIW, the most recent one LGTM.  I didn't object to your original
approach, though; I suppose you're waiting to hear from Chandler.