[lit] Make the 'llvm-lit' utility defend against a system where Python3

is the default.

The lit.cfg files are not all valid Python3 and I've no idea if anyone
is really prepared to update them. The easiest way I know of to ensure
that this script uses Python 2 is to use 'python2.7' in the command. Mac
and Linux are definitely fine with this and I think other platforms will
be as well, but if anyone struggles with this set up and has better
ideas, let me know.

llvm-svn: 229244