jit: Refine code generation

[vacantron]

The current operation handlers of JIT are generated by the template in src/rv32_template.c. However, it is not worthwhile to do the additional conversion since it just uses the one-to-one mapping by tools/gen-jit-template.py. It makes the readability worse and be hard to develop new features.

Considering replacing the current template with the common header file which the implementations could be configured during the compile-time might be the better way.

Related Issues:

• jit: Improve MMU efficiency in system simulation #500