kbuild: Consolidate header generation from ASM offset information

Largely redundant code is used in different places to generate C headers
from offset information extracted from assembly language output.
Consolidate the code in Makefile.lib and use this instead.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>