libgo: prune sighandler frames in runtime.sigprof

libgo: prune sighandler frames in runtime.sigprof

    When writing stack frames to the pprof CPU profile machinery, it is
    very important to insure that the frames emitted do not contain any
    frames corresponding to artifacts of the profiling process itself
    (signal handlers, sigprof, etc). This patch changes runtime.sigprof to
    strip out those frames from the raw stack generated by
    "runtime.callers".

    Fixes golang/go#26595.

    Reviewed-on: https://go-review.googlesource.com/126175

From-SVN: r263035