kbuild: move W=1 check for scripts/misc-check to top-level Makefile

This script is executed only when ${KBUILD_EXTRA_WARN} contains 1.
Move this check to the top-level Makefile to allow more checks to be
easily added to this script.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>