Build: Don't enable Neon SIMD exts with Armv6- When building for 32-bit Arm platforms, test whether basic Neon intrinsics will compile with the specified compiler and C flags. This prevents the build system from enabling the Neon SIMD extensions when targetting Armv6 and other legacy architectures that do not support Neon instructions. Regression introduced by bbd8089297862efb6c39a22b5623f04567ff6443. (Checking whether gas-preprocessor.pl was needed for 32-bit Arm builds had the effect of checking whether Neon instructions were supported.) Fixes #553