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

Use filechk to generate asm-offsets.h and generic-asm-offsets.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>
[ imported from Linux Kernel, commit 70a4fd6c56d0,
  with adjustment for U-Boot ]
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>