Add option to create bazel symlinks using relative paths (instead of absolute)

It'd be nice if the set of symlinks created by bazel (i.e. bazel-bin, bazel-genfiles, etc) had to option to use relative paths instead of absolute paths. In our case, this makes it easier to work off an NFS export and use OS-native tools on the files in that export.

Start by tracing where Bazel creates the bazel-bin and bazel-genfiles symlinks, then review the issue's NFS use case and the existing symlink behavior. Done means users can opt into relative symlink targets instead of absolute ones while preserving the current default; verify the behavior with the relevant existing tests.

Written by the indexing model from the issue text.