Add support for FreeBSD (#1148)

* Add support for FreeBSD (#22)

* update GNAT project to recognize -XOS=freebsd and configure accordingly
* add FreeBSD as a valid operating system
* add AMD64 as an alias for X86_64 because uname -m returns amd64 on FreeBSD
* add platform specific implementation for FreeBSD

To build on FreeBSD, we should use

gprbuild -j0 -P alr_env -XOS=freebsd

* Fixes requested during code review

Co-authored-by: GHA <actions@github.com>