How to add an assembly filter pass between cc1/cc1plus and the regular assembler

[Fred Fish]

Can someone please point me towards someplace in the gcc code or docs where I
can find information on adding a custom assembly language filtering pass between
cc1/cc1plus and the regular assembler?  Basically I have a standalone assembly
code level filter that I'd like to have run on the output of cc1/cc1plus before
it is passed on to the normal assembler.

-Fred