restored ISPC_FORCE_ALIGNED_MEMORY 

Fixed knc-i1x16.h to use

    #define ISPC_FORCE_ALIGNED_MEMORY 
       aligned load/stores
    #else
       unaligned load/store
    #endif

However, `__load<64>`/`__store<64>` will still be unaligned, unless ISPC_FORCE_ALIGNED_MEMORY is set, due to fails (see issue #632 & output of `grep knc::fails knc-i1x16.h`).