Fix inability to fully build pony on Raspberry PI 4's with 64-bit Raspbian

[chalcolith]

• Adds a Makefile variable pic_flag that you can set to either -fpic or -fPIC for both make libs and make configure
• On aarch64, the variable will default to -fPIC for make libs and make configure (see Can't build libponyc.tests on Raspberry PI 4 with Raspbian 64-bit #3870); on other Unix architectures it will be set to -fpic.
• Fixes Can't build libponyc.tests on Raspberry PI 4 with Raspbian 64-bit #3870

[chalcolith]

I haven't been able to figure out a way to detect that we're on raspbian from cmake or the shell; still investigating...

[ponylang-main]

Hi @kulibali,

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 3949.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.