weblist comes back blank on OSX when binary referenced in profile exists

Repro (needs to be on OSX, in `pprof` repo)

```
$ go test -o ./profile/foo -c -memprofile=mem.pb.gz ./profile/
$ (cd profile/ && ./foo -test.memprofile ../mem.pb.gz)
PASS
$ go tool pprof -http :6060 mem.pb.gz
```

In the browser window, navigate to the flame graph and click "Show source in new tab":


A completely empty source listing tab opens.

I spent way too much time digging into this today and found that this is related to whether the file referenced in the profile exists. That's right, you `rm profile/foo` and refresh the browser window, it will just work.

I didn't just try that out randomly but was motivated to try this after spending a few hours with the debugger attached.

When the file exists, we hit this code:

https://github.com/google/pprof/blob/a2290145206f1257bafcec3acc55ec994ef96c39/internal/binutils/addr2liner_nm.go#L63-L72

and `(*sourcePrinter).addStack` will get (from the `addr2LinerNM`) `plugin.Frame`s that have zero `File` and `Line`, i.e. the only thing in them is `Func`. Not only that, `Func` will also have an errant underscore prefix, which is an OSX thing (https://go-review.googlesource.com/c/go/+/196217). So basically it can never successfully look up which file a `Frame` is from. So at best we can expect a listing that doesn't reference the files.

But we don't even get that! We see why in this snippet as well. `sp.interest` is keyed by function names as stored in the profile, which don't have the underscore. But we look up with the underscore. So it never matches.

I hacked around this with a diff that also adds the underscore version into `sp.interest`. Then we get something like this (this is from when I had not ported my repro to use the `pprof` repo):


Meanwhile, interactive pprof's `list` command works flawlessly. When I looked at the source I realized why: `list` doesn't seem to share much code (if any at all) with weblist.

weblist is obviously much more powerful, so it would be good if it worked out of the box on OSX. Moving the file out of the way gives the basic listing, but disables the advanced functionality that's tied to having the binary present.

I have some more "meandering" (write-as-you-go) notes at https://gist.github.com/tbg/ff8de01920551be1a2193a2fd3ffd6b8, I don't think they're adding much but linking just in case.