How do write a gcc spec that adds to output files but does not append to command?

Question

I am porting gcc1.27 to a Tektronix 4404 from 1983. The assembler commands on this machine use +o=filename (no spaces) to specify output files.

So in gcc.c, the assembler spec of:

{!S:asm %{R} %{j} %{J} %{h} %{d2} %a %{gg:-G %g.sym}\
                      %g.s %{c:%{!o*:+o=%w%b.r}}%{!c:+o=%w%b.r}\n }

Results in "+o=foo.r" being added as an output (because of the %w marking it as output).

I want to be able to say "%w%b.r" to append to the output list, but not have that appended to the command.

Is there a known way of doing this or do I have to add my own new token?