js: additional check for _mm_mask_blend_epi32
js: additional check for _mm_mask_blend_epi32

Travis is now sometimes running on machines with AVX-512 support, which
is resulting is some compile-time errors for versions of clang which
don't support _mm_mask_blend_epi32.  This just checks for the builtin
used to implement _mm_mask_bleng_epi32 on clang before using it,
hopefully that's sufficient.

nemequ/simde,nemequ/simde,nemequ/simde