kpatch-gcc: split up object exclusion list
kpatch-gcc: exclude system_certificates.o

Fixes the following issue when running kpatch-build against the latest
upstream kernel:

    system_certificates.o: no changed functions were found
kpatch-build: don't link unchanged objects

The FILES variable can contain unchanged objects, so don't use it to
determine which objects to link.  Instead, just use all the objects that
were placed in the output directory by create-diff-object.
Merge pull request #349 from jpoimboe/system_certificates

upstream kernel fixes