Always translate global objects as static mut

[lucic71]

This is a continuation of #65:

This is incomplete, it only solves part of the problem.

In Rust, static global variables require Sync so that multiple threads can take an immutable reference to the static storage, static mut global variables don't require Sync because it's by itself unsafe to access the static mut storage.

This PR solved the problem for static global struct objects that contained raw pointers. Raw pointers are not Sync in Rust so any struct containing them needs to implement the Sync trait. What this PR does not solve is global arrays containing pointers in static storage, for example [*const T; N].

One solution would be to translate the const global variables as const instead of static. However const global variables don't have a stable address in the final binary. Other solution would be to wrap the array in a newtype, but this makes later usages of the array harder in the codegen.

None of the two above solution is complete. For the moment we can translate all global variables as static mut instead of static so that we don't have to implement Sync for them. By construction, the generated code is guaranteed not to modify the global variable, even if it's declared as static mut instead of static.

This only touches the unsafe model.