MFC r346085: Add Cirrus CI config file to support CI builds

Also followup commits:
r346121: Cirrus-CI: pass OVMF env var to test script for upcoming changes
r350302: cirrus.yml: use OVMF.fd from uefi-edk2-qemu-x86_64 package
r350449: cirrus.yml: stop fetching OVMF.fd now that we're using the pkg
r356867: Cirrus-CI: bump VM image to FreeBSD 12.1

Sponsored by: The FreeBSD Foundation