Thanks for the hints; I'm building a kernel with the normal flags
(took cpuflags out of mk.conf).  I of course want -O2, so putting that
back seems like a better choice for me than fixing the bootloader.
(Plus, this machine is headed to 1.6 anyway.)

The problem seems to be in the Makefile in src/sys/arch/sparc/compile/FOO,
where

COPTS?= -O2

If this were +=, things would probably be ok.
But, then it would be hard to override optimization (without editing
the makefile).  But perhaps that is ok.

When you say 'fixed in -current', do you mean that big kernels can be
loaded, or that -O2 and cpuflags interact as one would like for kernel
builds?