[llvm-readobj] Fix output interleaving issue caused by using multiple streams at the same time.

Summary:
Use llvm::fouts() as the default stream for outputing. No new stream
should be constructed to output at the same time.

https://bugs.llvm.org/show_bug.cgi?id=42140

Reviewers: jhenderson, grimar, MaskRay, phosek, rupprecht

Reviewed By: rupprecht

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63115

Patch by Yuanfang Chen!

llvm-svn: 363198