x/telemetry/crashmonitor: fixup inaccurate locations in sigpanic stacks

[findleyr]

As described in #70637 (comment), the current behavior of crash output isn't quite correct for usage with runtime.CallersFrames: for runtime.sigpanic frames, we need to increment the PC of the next frame to get the correct location.

This issue tracks implementing this workaround in x/telemetry/crashmonitor.

In the future, we hope that debug.SetCrashOutput will support structured output, so that we can remove this fix.

[gabyhelp]

Related Issues

• runtime: panic crash should not show top panic frame #17901 (closed)

Related Code Changes

• runtime, runtime/pprof: add Frames to get file/line for Callers
• runtime: avoid bad unwinding from sigpanic in C code

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