Do yourself a service and write the Scheme compiler in Scheme itself, doing direct assembly code generation.

http://scheme2006.cs.uchicago.edu/11-ghuloum.pdf

Also a nice way to learn about bootstraping compilers.

Generating code code for another OS without Scheme support is just a matter of having your compiler cross-compile to the other OS. Then use the fresh backed compiler to re-compile your compiler in the new OS.