swscale: partially move the arch specific code left

PPC and x86 code is split off from swscale_template.c. Lots of code is
still duplicated and should be removed later.

Again uniformize the init system to be more similar to the dsputil one.

Unset h*scale_fast in the x86 init in order to make the output
consistent with the previous status. Thanks to Josh for spotting it.