MATLAB Coder can generate C code that can be compiled by whatever toolchain you want. However MATLAB Coder supports very little graphics, and especially in your release does not support App Designer graphics.
Copying the m code from app designer to 2015 cannot possibly work without copying all of the app designer classes as well. Even that might not be enough as I believe that app designer relies on kernel graphics changes that would not be part of the R2015b runtime. It is unlikely to work.
It would probably be less expensive to upgrade the 32 bit system to 64, or to use the App Compiler to generate code to run on a server that the 32 bit system would connect to using a web browser. (Note that this is not recommended for public access as the code is not hardened against security problems.)
