Optimize backtrace code for perf PMI handler.

[Andi Kleen]

While doing perf record -g I regularly exceed the time quote for the NMI:

This leads to perf shortening the period, which causes various problems.

This patchkit optimizes two sources of longer latencies in the NMI backtrace
code. There's probably more work needed to fix other sources of latencies
too.

-Andi

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