arch-arm: Fix some poorly done type max and min in NEON

arch-arm: Fix some poorly done type max and min in NEON

The ISA code for ARM calculates min and max elements for types using
bit manipulation. That triggers some warnings, treated as errors, as
the compiler can tell that there is an overflow and the sign
flips. Fixed using standard lib definitions instead.

Change-Id: Ie2331b410c7f76d4bd87da5afe9edf20c8ac91b3
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3481
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>