target/arm: Rename TBFLAG_A64 ZCR_LEN to VL

target/arm: Rename TBFLAG_A64 ZCR_LEN to VL

With SME, the vector length does not only come from ZCR_ELx.
Comment that this is either NVL or SVL, like the pseudocode.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220607203306.657998-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>