target/arm: Explicitly enable VFP short-vectors for aarch32 -cpu max

target/arm: Explicitly enable VFP short-vectors for aarch32 -cpu max

At the moment our -cpu max for AArch32 supports VFP short-vectors
because we always implement them, even for CPUs which should
not have them. The following commits are going to switch to
using the correct ID-register-check to enable or disable short
vector support, so we need to turn it on explicitly for -cpu max,
because Cortex-A15 doesn't implement it.

We don't enable this for the AArch64 -cpu max, because the v8A
architecture never supports short-vectors.

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