kbuild: Really don't clean bounds.h and asm-offsets.h

Commit 7d3cc8b6

 tried to keep bounds.h and asm-offsets.h during make
clean by filtering these out of $(clean-files), but they are listed in
$(targets) and $(always) and thus removed automatically. Introduce a new
$(no-clean-files) variable to really skip such files in Makefile.clean.

Signed-off-by: Michal Marek <mmarek@suse.cz>