toolchain: add a USE_MMU build option

Some packages need to know whether the MMU is in play, so add a toolchain
option to expose the feature to them.

[Peter: only show option on archs where it makes sense]
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>