Add a Cmake generic build option and remove x86_32 build capabilities.

Adds the ability to build our generic build option without manually enabling it in the cmake file.
If trying to build on a x86_32 host, throw a fatal error but say that a binary can be built as a generic build if one wants.
Merge pull request #418 from Sonicadvance1/x86_32-cmake-removal

Add a Cmake generic build option and remove x86_32 build capabilities.