Enable USE_SSEVECT only on x86_64 with SSE

Patch enables USE_SSEVECT (vector implementation based on SSE
intrinsics) only on a proper hardware, i.e, x86_64 with SSE.

For the rest of architectures (armv7hl, aarch64, power8, etc.) use
USE_EXTVECT.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
why is ``` defined(__SSE__) ``` alone not enough, is it an extra sanity check?