traced_perf: Include kernel return addresses in stack traces

[pcc]

These are sometimes useful for identifying the call site (e.g. in case of an indirect call or multiple calls to the same function).

[pcc]

P.S. To investigate my actual scenario I also had this diff:

-    frame.function_name = std::move(function_name);
+    frame.function_name = std::move(function_name) + " " +
+                          base::Uint64ToHexString(sample.kernel_ips[i]);

so that the flame graph entries would be discriminated by call site. We may consider adding an option to the UI to do something similar, which may also be useful for userspace stack traces.