llext: add llext_peek()

The only way so far to access extension images is via a memory
buffer. Since this, supposedly, will also be a rather common method,
it makes sense to add a method to access extension data quickly by
obtaining a pointer instead of copying data into local buffers. Add a
llext_peek() method for that.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>