ENH: tell gcc to unroll strided copy loops

The strided copy loops profit a lot from unrolling as the number of
operations executed is in each iterations very small.
GCC needs to be told explicitly to do unrolling even on O3.

Unrolling is only profitable if the move can be done in a single
instruction, else the increased code size makes it slower, thus the flag
is only used for operations on element sizes less equal the native
pointer size.

Tested to improve performance of by 20-50% on intel core2duo, xeon
5xxx/7xxx and amd phenom x4.