Help, I killed my machine.

[Ben Timby]

I was upgrading from 4.10-STABLE to 5.4-STABLE, following the 
instructions in the freebsd handbook and something went wrong.

I used CVSup to update my sources. I built the world and kernel as follows:

cd /usr/src
make buildworld && make buildkernel

I am using the GENERIC config. I had to copy 
/usr/src/sys/i386/conf/GENERIC.hints to /boot/device.hints. Before the 
above builds worked properly.

I installed the kernel, and rebooted the system.

It booted (mostly) ok, sudo did not work properly, so I had to login as 
root. I did mergemaster -p. I had to add the new proxy user and group 
for pf. After this, I did:

cd /usr/src/
make installworld

during the process, it died in:

/usr/src/bin/test

with Signal 12.

No commands worked after this point, All I received was Signal 12. I 
cannot boot into single user mode, I receive a Signal 12 from any shell 
I try to use.

I understand a Signal 12 is a non-existant system call. The half 
installworld probably caused this.

How can I recover from this?

Thanks.