What do you do if a kernel dumped core but you did not expect it, and it is therefore not compiled using config -g? Not everything is lost here. Do not panic!
Of course, you still need to enable crash dumps. See above on the options you have to specify in order to do this.
Go to your kernel config directory (/usr/src/sys/arch/conf) and edit your configuration file. Uncomment (or add, if it does not exist) the following line
makeoptions DEBUG=-g #Build kernel with gdb(1) debug symbols
Rebuild the kernel. Due to the time stamp change on the Makefile, there will be some other object files rebuild, for example trap.o. With a bit of luck, the added -g option will not change anything for the generated code, so you will finally get a new kernel with similar code to the faulting one but some debugging symbols. You should at least verify the old and new sizes with the size(1) command. If there is a mismatch, you probably need to give up here.
Go and examine the dump as described above. The debugging symbols might be incomplete for some places, as can be seen in the stack trace in the example above where some functions are displayed without line numbers and argument lists. If you need more debugging symbols, remove the appropriate object files and repeat the kgdb session until you know enough.
All this is not guaranteed to work, but it will do it fine in most cases.