CI/ASAN build failure with -fundefined

As of https://github.com/envoyproxy/nighthawk/pull/587 the asan build fails when `-fundefined` is passed with the following error:

```
ld.lld: error: undefined symbol: __muloti4
```

This seems related to https://github.com/envoyproxy/envoy/issues/13973, which was introduced via an update to
`absl`, and eventually was resolved in Envoy by bumping `absl`. Yet this error persist in Nighthawk.

I've spend some time trying to figure out how to address this, without coming up with a good solution yet.
To unblock, `-fundefined` gets dropped from [.bazelrc](https://github.com/envoyproxy/nighthawk/blob/master/.bazelrc) in https://github.com/envoyproxy/nighthawk/pull/587.

Filing this issue to track status of re-enabling `-fundefined`