microblaze: add stack unwinder

This has been baking for awhile, I think it's ready for prime time. I've beat on it in a noMMU kernel for awhile - backtracetest module, kmemleak, dump_stack() in a number of different situations - all look good. Thanks to Paul Mundt for initial advice on this feature.

I don't have MMU hardware or the ability to build a MMU kernel, so all I've done there is check for gross compile errors. The MMU kernel handles kernel stacks differently, which isn't an issue except in the trap handlers (interrupt, syscall, etc.). This patch should work for MMU kernels, but they won't trace back beyond a system trap. The intent is to provide hooks for someone to add that capability later.

The key feature of the design is a search backwards from each return address for the instruction that creates a stack frame. CONFIG_FRAME_POINTER causes Microblaze gcc to use r19 as a frame pointer, but because previous frame pointers are not stored in a consistent location within each frame, it's not possible to avoid some sort of search to determine frame size. (Side note, since -fno-omit-frame-pointer isn't useful for Microblaze, it would be nice if Microblaze could "opt-out" of FRAME_POINTER when selecting FUNCTION_TRACER).

A brute-force search was chosen over a kallsyms-assisted search (as used in the sh architecture) to avoid going off the rails in cases where kallsyms chooses the closest global address instead of the true (local) function address. Obviously there's an accuracy/speed tradeoff, but under an assumption that most functions are reasonably-sized and most backtraces are not performed in time-critical paths, this should be OK.

Happy unwinding,
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Steven J. Magnani "I claim this network for MARS!
www.digidescorp.com Earthling, return my space modulator!"

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