target/arm: Implement v8M VLLDM and VLSTM

For v8M the instructions VLLDM and VLSTM support lazy saving
and restoring of the secure floating-point registers. Even
if the floating point extension is not implemented, these
instructions must act as NOPs in Secure state, so they can
be used as part of the secure-to-nonsecure call sequence.

Fixes: https://bugs.launchpad.net/qemu/+bug/1768295


Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180503105730.5958-1-peter.maydell@linaro.org