kbuild: fix AFLAGS use in h8300 and m68knommu

In most cases when AFALGS is manipuled direct this is a bug
and EXTRA_AFLAGS should have been used.
Fix the obvious candidates.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Greg Ungerer <gerg@uclinux.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>