If you want to use a toolchain file there is an easier solution (IMHO) than what is proposed by @auledoom. You do not need to write the shell wrapper scripts at all, simply put this in the toolchain file:
# the name of the target operating system
set(CMAKE_SYSTEM_NAME Linux)
# Which compilers to use for C and C++
set(CMAKE_C_COMPILER gcc -m32)
set(CMAKE_CXX_COMPILER g++ -m32)
This will make it a “list variable” in cmake. This solution works for me. Benefit of the toolchain file is that you can there also define paths for 32bit libraries etc, which is usually different from standard paths.