Compile sun2 kernel with -fno-omit-frame-pointer for GCC8 as a work
around for reproducible kernel freezes just after ``Starting postfix.'',
where I cannot even enter DDB nor obtain crash dump.

I still haven't figured out why. Possibly something wrong with -Os
optimization level for GCC/m68k, cf.,
http://mail-index.netbsd.org/port-sun3/2020/07/19/msg000166.html

(rin)