target-arm: Don't permit ARMv8-only Neon insns on ARMv7

The Neon instructions VCVTA, VCVTM, VCVTN, VCVTP, VRINTA, VRINTM,
VRINTN, VRINTP, VRINTX, and VRINTZ were only introduced with ARMv8,
so they need a guard to make them UNDEF if the CPU only supports ARMv7.
(We got this right for all the other new-in-v8 insns, but forgot
it for these Neon 2-reg-misc ops.)

Reported-by: Christophe Lyon <christophe.lyon@linaro.org>
Tested-by: Christophe Lyon <christophe.lyon@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1465492511-9333-1-git-send-email-peter.maydell@linaro.org