Changes to support Intel AVX/AVX2 in cvResize().

This patch
- adds code path for Intel AVX2 detection, and
- utilizes Intel AVX/AVX2 intrinsics within cvResize()

check_regression=*esize*
That is correct. /arch:AVX and /arch:AVX2 allow the compiler to use extended instructions in generated code, even when the intrinsics aren't used. -mavx and -mavx2 on GCC have the same effect, as well.

Easiest solution would be to consolidate AVX/AVX2 functions in separate files, and specify /arch:AVX* or -mavx* only when compiling those files. Should I refactor the code and submit a new pull request?