postproc: Replaced inline asm for prefetching with prefetch functions

Prefetching functions are defined in postprocess_template using the
RENAME macro so that prefetching is used when available. For x86
targets inline asm is used and the functions are non-empty only for
cpus where prefetching is available. For non x86 targets the gcc bultin
prefetch is used if it is available, otherwise no prefetching is done.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>