Fix link errors on macOS Big Sur

[ergl]

With the change from Catalina to Big Sur, Apple moved the location of
libraries. On Big Sur, it is required to pass the location of the System
library. The -lSystem option is still required for macOS 10.15.7 and
lower.

We don't currently distinguish between different macOS versions, so we
should keep both options. As long as the absolute path of the System
library is present, the -lSystem option won't pose any problems on Big
Sur.

This should fix #3684.

[ergl]

As for keeping -lSystem around: ponyc failed to link on Mojave if I removed the option.

[ponylang-main]

Hi @ergl,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3686.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

Thanks @ergl.