target-arm: Support v6 barriers in linux-user mode

target-arm: Support v6 barriers in linux-user mode

ARMv6 implemented various operations as special cases of cp15 accesses
which are true instructions in v7; this includes barriers (DMB, DSB, ISB).
Catch this special case at translate time, so that it works in linux-user
mode (which doesn't provide a functional get_cp15 helper) as well as
system mode.

Includes minor cleanup of the existing cases (single switch statement,
and doing the "OK in user mode?" test explicitly rather than hiding it in
cp15_user_ok()).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>