Cirrus-CI: use makefs if root size exceeds QEMU's virtual FAT limit

We use QEMU's virtual FAT support to avoid having to create a disk
image because it is much faster, but it has a limit of about 500MB.
Artifacts produced by the GCC 12 CI job exceeded this size.

Add support for creating a FAT partition image and MBR-partitioned disk
image and use it when the file system is too large for QEMU.

In one run the Cirrus-CI LLVM test task took 1m33s using QEMU's virtual
FAT while the GCC task took 6m48s using makefs+mkimg.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38589