arch-arm: Fix build errors with gcc 10.2

arch-arm: Fix build errors with gcc 10.2

The "-Werror=type-limits" flag in GCC 10.2 reports these errors,
because ``imm`` in neon.isa, and ``imm`` and ``count`` in sve.isa are
unsigned, and they're used to do ``imm < 0`` and ``imm * count >= 0``
comparison.

Change-Id: I33934357f578a9fc1040a6d9c08ea929fb36eb47
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33154
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>