[libc++] Improve Python 3 compatibility for merge_archives.py

Popen.communicate() method in Python 2 returns a pair of strings, and in
Python 3 it returns a pair of byte-like objects unless universal_newlines
is set to True. This led to an error when using Python 3. With this patch,
merge_archives.py works fine with Python 3.

Thanks to Sergej Jaskiewicz for the patch.
Differential Revision: https://reviews.llvm.org/D66649

llvm-svn: 369764