target/arm: Split out make_svemte_desc

target/arm: Split out make_svemte_desc

Share code that creates mtedesc and embeds within simd_desc.

Cc: qemu-stable@nongnu.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Gustavo Romero <gustavo.romero@linaro.org>
Message-id: 20240207025210.8837-5-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 96fcc9982b4aad7aced7fbff046048bbccc6cb0c)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>