Extending NaplesPU for 64-bit support

nu+ toolchain can be extended to support 64-bit operations. A git branch with full 64-bit support is provided. Consequently, if it is necessary to compile the toolchain supporting this extension, a checkout on llvm-7 branch is required.

Changes are related to both frontend and backend.

nu+ frontend modifications

nu+ frontend abstracts target informations through the TargetInfo class, extending it into the NuPlusTargetInfo implementation.