[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