Upgraded to 2.1.55. The boot loader now uses OF instead of hardware poking. Upgraded to Linux 2.1.53 and 2.1.54 and applied Miquel van Smoorenburg's serial console patches. Shutdown reboots the machine. The MESH SCSI driver detects all my SCSI devices.
OpenPIC interrupts work fine now.
8259 interrupts mainly work, except for some problems with the keyboard and the serial port. I ported the MESH SCSI driver for the PowerMac to my machine, but it has some problems with the SCSI device detection. OpenPIC interrupts seem to work, but legacy 8259 interrupts still don't come through. The kernel boots until the moment to mount the root filesystem. Interrupts still don't come through.