'cat' command for internal shell - Support Python 3

LLVM Bug Id : 36449

Revision 328563 caused tests to fail under python 3.

This patch modified cat.py file to support both python 2 and 3.
This patch also fixes CRLF issues on Windows.

Patch by Chamal de Silva

Differential Revision: https://reviews.llvm.org/D45077

llvm-svn: 329123