Create lib64 symlink for 64-bit, non-macOS, POSIX environments (#3584)
## Summary
Closes
https://github.com/astral-sh/uv/issues/3578#issuecomment-2110675382.
## Test Plan
Verified that in the OpenSUSE test, we create both, and they're
symlinks:
```text
INFO: Creating virtual environment with `venv`...
INFO: Installing into `venv` virtual environment...
DEBUG Found a virtualenv named .venv at: /tmp/tmp4nape29h/.venv
DEBUG Cached interpreter info for Python 3.10.14, skipping probing: .venv/bin/python
DEBUG Using Python 3.10.14 environment at .venv/bin/python
DEBUG Trying to lock if free: .venv/.lock
purelib: "/tmp/tmp4nape29h/.venv/lib/python3.10/site-packages"
platlib: "/tmp/tmp4nape29h/.venv/lib64/python3.10/site-packages"
is_same_file(purelib, platlib): Ok(true)
```