Remove QVM_COMPAT, allow both QVM and NaCl ABI at the same time.

* Made GameVM an abstract class implemented by QVMGameVM and NaClGameVM
* The QVM native VMs are now name-qvm-native.so
* NaCl compilation options are visible but off by default

Still TODO:
* Better reporting of errors when creating VMs
* Add a prefix to NaCl native VMs
Remove the last pieces of the LLVM VMs, we have NaCl now
Fix the compilation of native NaCl VMs in C++