Add a basic clang-format configuration file

This gets reasonably close to the existing format in sys/kern but will
probably require some changes to upstream clang-format before it can be
used as the default formatting tool.

I tried formatting a few files in sys/kern and the result is pretty close to
the existing code. However, this configuration file is not ready to be used
without manually checking the output.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D20533