s390/nospec: build expoline.o for modules_prepare target

When CONFIG_EXPOLINE_EXTERN is used expoline thunks are generated
from arch/s390/lib/expoline.S and postlinked into every module.
This is also true for external modules. Add expoline.o build to
the modules_prepare target.

Fixes: 1d2ad084

 ("s390/nospec: add an option to use thunk-extern")
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Tested-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Tested-by: C. Erastus Toe <ctoe@redhat.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Link: https://lore.kernel.org/r/patch-1.thread-d13b6c.git-a2387a74dc49.your-ad-here.call-01656331067-ext-4899@work.hours
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>