The kunit.py utility builds an ARCH=um kernel and then runs it.  Add
optional --make_options flag to kunit.py allowing for the operator to
specify extra build options.

This allows use of the clang compiler for kunit:
  tools/testing/kunit/kunit.py run --defconfig \
    --make_options CC=clang --make_options HOSTCC=clang
Signed-off-by: NGreg Thelen <gthelen@google.com>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Reviewed-by: NNathan Chancellor <natechancellor@gmail.com>
Tested-by: NDavid Gow <davidgow@google.com>
Signed-off-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>