fix: force use of libc++ with bazel --config=ubsan (#4416)
fix: force use of libc++ with bazel --config=ubsan (#4416)

The `-fsanitize=undefined` linkopt and 128-bit multiplication don't
play well together when using libstdc++, producing a link error:
```
error: undefined reference to '__muloti4'
```

So, use libc++ instead.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp