[common libraries] --log_backtrace_at flag not hooked up

This is a very useful flag for debugging, can get a backtrace on demand from a triggering VLOG statement. Right now it doesn't seem to be hooked up in our source. No Abseil C++ logging as of yet, but since we didn't seem to really "need" this for so long, a little unclear how to prioritize, but I wanted to file a bug to track nonetheless.

Ideally we'd have `--log_backtrace_at=file.cc:50` flag trigger the backtrace when that VLOG statement in file.cc line 50 was logged if we do want to patch this up.

https://github.com/google/xls/blob/56a46a1b6c66d30840dae24b2ad61d5ae763ea5f/xls/common/logging/log_message.cc#L334