Support other architectures

[arames]

Rejit currently only supports x86_64.

One option is of course to port to other architectures one by one.

An alternative is to add C++ architecture-independent support (still using rejit's mechanisms like fast-forwarding), where JIT code could be implemented piece by piece to replace performance critical sections. Like the first option this requires a significant effort, but would have the advantage of being functional on all architectures.