ci: Add FreeBSD CI

We use 'freebsd-11' instead of 'freebsd' as tags here because newer
FreeBSD versions can include API changes which are worth testing.
7 jobs for wip/lantw/ci-on-freebsd in 22 minutes and 1 second (queued for 5 seconds)
latest
Status Job ID Name Coverage
  Build
passed #53871
cross-android_api21_arm64

00:01:20

passed #53872
cross-mingw64

00:02:10

passed #53870
fedora-x86_64

00:03:30

passed #53875
freebsd-11
freebsd-11-x86_64

00:02:03

passed #53873
win32
msys2-mingw32

00:08:03

passed #53874
win32
vs2017-x64

00:05:35

 
  Coverage
passed #53876
coverage

00:01:20

77.0%