target/arm: Pass env pointer through to gvec_bfdot helper

target/arm: Pass env pointer through to gvec_bfdot helper

Pass the env pointer through to the gvec_bfdot helper,
so we can use it to add support for FEAT_EBF16.

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