linuxkpi: Provide a non-NULL value for THIS_MODULE

THIS_MODULE is used to differentiate modules on Linux. We currently
completely stub out any Linux struct module usage, but THIS_MODULE
is still used to populate the "owner" fields of various drivers.
Even though we don't actually dereference these "owner" fields they
are still used by drivers to check if devices/dmabufs/etc come
from different modules. For example, during DRM GEM import some
drivers check if the dmabuf's owner matches the dev's owner. If
they match because they are both NULL drivers may incorrectly think
two resources come from the same module.

This adds a general purpose __this_linker_file which will point to
the linker file of the module that uses it. We can then use that
pointer to have a valid value for THIS_MODULE.

Reviewed by: bz, jhb
Differential Revision: https://reviews.freebsd.org/D44306