target/arm/sve_helper: Fix compilation with clang 3.4

Clang 3.4 does not know the "flatten" attribute yet. We've already
introduced the QEMU_FLATTEN macro for this in commit 97ff87c0,
so use this macro now here, too, to fix this issue.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1543399094-2260-1-git-send-email-thuth@redhat.com
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>