kbuild: merge bounds.h and asm-offsets.h rules

The rules "cmd_bounds" and "cmd_offsets" are almost the same.
(The difference is only the include guards.)  They can be merged.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>