target/arm: Convert PMULL.8 to gvec

target/arm: Convert PMULL.8 to gvec

We still need two different helpers, since NEON and SVE2 get the
inputs from different locations within the source vector.  However,
we can convert both to the same internal form for computation.

The sve2 helper is not used yet, but adding it with this patch
helps illustrate why the neon changes are helpful.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200216214232.4230-5-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>