[llvm-readobj] Fix output interleaving issue caused by using multiple streams at the same time.
Use llvm::fouts() as the default stream for outputing. No new stream
should be constructed to output at the same time.
Reviewers: jhenderson, grimar, MaskRay, phosek, rupprecht
Reviewed By: rupprecht
Differential Revision: https://reviews.llvm.org/D63115
Patch by Yuanfang Chen!