create_spdx: ensure is_work_shared() is unique

There is a function with the same name is_work_shared() in the archiver class
this causes a conflict when both classes are included. Use work-shared as the
check in WORKDIR to allow for other packages beyond the kernel and gcc that
use a common shared-work source directory.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>