[llvm-objcopy, tests] Fix several llvm-objcopy tests

Summary: In Python 3, sys.stdout.write expects a string rather than bytes. In order to be able to write the bytes to stdout, we need to use the buffer directly instead. This change is borrowing the implementation for writing to stdout that cat.py uses. Note that we cannot use cat.py directly because the file we are trying to open is a gzip file.

Reviewers: asmith, bkramer, alexshap, jakehehrlich

Reviewed By: alexshap, jakehehrlich

Subscribers: jakehehrlich, llvm-commits

Differential Revision: https://reviews.llvm.org/D49515

llvm-svn: 337567