Don't use vdotq_s32 if it's not available

[Ronsor]

dotprod extensions aren't available on some ARM CPUs (e.g. Raspberry Pi 4), so check for them and only use them if they're available.

Reintroduces the code removed in 84d9015 if __ARM_FEATURE_DOTPROD isn't defined.

Fixes my and other problems reported in the comments of #58.