nios2: remove empty files from nios2e-zephyr

Use a kbuild trick to force built-in.o to get built even if there isn't
any code so we can link properly.  This is cleaner than keeping around
empty files that don't do anything.

Change-Id: I4214d21104fe5f49613fa5697c8116b0e8c8aa50
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>