Added FreeBSD workflow in GitHub Actions

[Semphriss]

Thanks to VMActions, it is possible to run CI builds for FreeBSD and Solaris and soon OpenBSD and NetBSD too.

The current build system is functional, but does not generate/upload any artifact and only covers FreeBSD.

[Semphriss]

Cancelling the workflow, FreeBSD freezes during the execution of CMake:

-- The C compiler identification is Clang 10.0.1
-- The CXX compiler identification is Clang 10.0.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Check if the system is big endian
-- Searching 16 bit integer
-- Looking for sys/types.h

It seems to freeze always at that place. I'll investigate later (help would be appreciated 🙏). I won't cancel the workflows just yet, but I'll cancel them if e. g. tomorrow they didn't either fail or unblock.