Make template app compilable both to circuits and x86

[NickVolynkin]

Developers who start working with zkllvm often want to see a simple application that compiles both to a normal amd64 application and to a circuit with zkllvm.

Otherwise it's hard to understand how to make an app that compiles in both ways and how to run tests on it.

DoD:

• Simple app with main.
• Unit-tests for this app.
• (optionally) functional test for a compiled app.
• Cicruit compilation

[nkaskov]

We've recently created the desired functionality while doing this issue: NilFoundation/zkLLVM#181
So, once this PR is merged, we can bring such an example in the template repository: NilFoundation/zkLLVM#209

[nkaskov]

We need to modify CMakeLists in src directory so it can be used to compile to x86. This will also require to fix the code and add main similar to how we do it in zkLLVM tests.