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

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

This commit is mostly inspired by the following commit of Linux.

    commit 39664e2f3cdef98f42437e903159a6044a1d99d6
    Author: Masahiro Yamada <yamada.m@jp.panasonic.com>
    Date:   Mon Jan 5 15:57:15 2015 +0900

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

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>