[opt-viewer] Include default values in help output

Summary:
Python's argparse module includes a `%(default)s` format specifier that
can be used to print the default value of an option in its help text.
Use this for opt-viewer utilities' `--jobs` arguments.

Reviewers: anemet

Reviewed By: anemet

Subscribers: llvm-commits, fhahn

Differential Revision: https://reviews.llvm.org/D34081

llvm-svn: 305155