kunit: add --make_options

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: Greg Thelen <gthelen@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: David Gow <davidgow@google.com>
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>