target/arm: Implement MVE LCTP

Implement the MVE LCTP instruction.

We put its decode and implementation with the other
low-overhead-branch insns because although it is only present if MVE
is implemented it is logically in the same group as the other LOB
insns.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210614151007.4545-7-peter.maydell@linaro.org