kbuild: move modpost out of 'scripts' target

I am eagar to build under the scripts/ directory only with $(HOSTCC),
but scripts/mod/ highly depends on the $(CC) and target arch headers.
That it why the 'scripts' target must depend on 'asm-generic',
'gcc-plugins', and $(autoksyms_h).

Move it to the 'prepare0' stage. I know this is a cheesy workaround,
but better than the current situation.

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