[Crash-utility] [RFC v2] Use register value in elf note NT_PRSTATUS to do backtrace

Hi Dave and list, I guess you still remember that a few weeks ago Wen has sent a patch which intends to use register values in NT_PRSTATUS notes for backtracing if no panic task was found. Thanks for your review and the suggestions are very useful. However, Wen was busy with other work for these days, so I'll continue with this work and now the 2rd version patch is attached. v2: Address review feedbacks. 1) Set up a flag in pc->flags2 and it's determined during the diskdump file init procedure. 2) Seperate code according to the that flag. 3) Also, we've done some tests on dumpfile of xen kernel and the trouble described in the previous mail was gone. So we're looking forward to your feedback and if you still have any problems with it, please tell us. Thanks, Wang Chao