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

(cherry picked from commit 9ce187bf83c0593d9639662417923f0e054c19d3)

Cirrus-CI: use FreeBSD 12.2 image

Direct commit to stable/12 as main has other changes here, using the
llvm package to avoid building the toolchain.

(cherry picked from commit 046f91f21ac619f7307a8e853f863ded4df7f322)

Cirrus-CI: avoid boot smoke test on stable/12

The boot smoke test boots from a FAT filesystem (created in-memory
by QEMU), but this does not work on stable/12:

mountroot: waiting for device /dev/ada0s1...
Mounting from msdosfs:/dev/ada0s1 failed with error 19.

For now on stable/12 use Cirrus-CI as only a build test.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 05b9673637feedb3ef0cf51fd9b7e914b08f7554)

Cirrus-CI: Add descriptive task name

Previously it appeared only as "main" in places like GitHub's list
of checks run as part of a pull request.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 903526542ac2309c08c769e517ea173a9f67cdb2)

"boot smoke test" dropped from task name for the MFC, because Cirrus
runs a build test only on stable/12 (see 05b9673637fe for details).

(cherry picked from commit 20c918af7a3de84a6fc7d840a712fc4aa4dcdb69)