embedded: Allow to disable the prefetching.

Disabling the prefetching is a win on arm9 (arm926ejs, e.g. TI
Davinci DM644x). This was benchmarked using the simple Bench.st
benchmark.

2013-04-28  Holger Hans Peter Freyther  <holger@freyther.de>

	* libgst/gstpriv.h: Allow to disable the DO_PREFETCH.
embedded: Allow to disable the line number bytecode

This is more noticable on smaller devices. Allow to disable the
line number bytecode. This changes the runtime of Bench.st for
4 iterations from 26s to 19s.