kbuild: Don't reset timestamps in include/generated if not needed

Use filechk to generate asm-offsets.h and bounds.h. Based on a patch by
Valdis Kletnieks.

Reported-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Acked-By: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>