Add kernel support for oprofile callgraphs on AVR32

This patch adds backtracing capability to oprofile profiling in kernel
and user mode on AVR32. This is done by going through the frames on the
stack and adding oprofile traces for all return addresses. The code
being profiled has to be compiled with frame pointers to make this work.

Signed-off-by: Nikolaus Voss <n.voss@weinmann.de>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>