The buildsystem passes ARCH="$(ARCH)" by default, but when building kernel modules
we need to pass ARCH="$(LINUX_KARCH)" since Linux uses differet arch names in Kbuild.
Fixes ipset build on brcm47xx (mipsel -> mips) and likely others.
git-svn-id: svn://svn.openwrt.org/openwrt/packages@31468 3c298f89-4303-0410-b956-a3cf2f4a3e73