target/arm: Fix sign-extension in sve do_ldr/do_str

target/arm: Fix sign-extension in sve do_ldr/do_str

The expression (int) imm + (uint32_t) len_align turns into uint32_t
and thus with negative imm produces a memory operation at the wrong
offset.  None of the numbers involved are particularly large, so
change everything to use int.

Cc: qemu-stable@nongnu.org (3.0.1)
Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>