Create lib64 symlink for 64-bit, non-macOS, POSIX environments

[charliermarsh]

Summary

Closes #3578 (comment).

Test Plan

Verified that in the OpenSUSE test, we create both, and they're symlinks:

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)

[konstin]

Code lgtm but it's odd because virtualenv doesn't create this directory for me

[charliermarsh]

Would you vote against merging? I tend to view venv as the reference implementation but also agree it's strange.

[konstin]

The extra directory wont hurt, but it would be good to understand what virtualenv does here

[charliermarsh]

@gaborbernat - sorry to bother, have you seen this before?

[gaborbernat]

Could be bug, I know Fedora creates two folders and should work.