neon/cvt: disable some code on 32-bit x86 which uses _mm_cvttsd_si64
neon/cvt: disable some code on 32-bit x86 which uses _mm_cvttsd_si64

This function is only available in 64-bit mode.  Unfortunately there
really isn't a good way to emulate it, at least as far as I know.

Fixes #876

nemequ/simde,nemequ/simde,nemequ/simde