Some m68k variants do have a MMU so the architecture can set
ARCH_USE_MMU. That means we can have a m68k-unknown-linux-gnu
configuration and that m68k-unknown-uclinux-uclibc needs to select
LIBC_UCLIBC_NG since it's no longer the default.
Fixes#2040
Signed-off-by: Chris Packham <judge.packham@gmail.com>
