Refactor code generations

[jserv]

Both src/arm-codegen and src/riscv-codegen.c have some functions in common. The code generation can be refactored with the following changes:

1. split the shared functions/variables to new file src/codegen.c from src/{arm,riscv}-codegen.c
2. In the end of src/codegen.c, there should be a statement `#include "src/arch-codegen.c" which links to Arm or RISC-V code generation implementation.
3. The generated shecc executable file should be capable of showing its configurations such as the supported architecture and ABI.

[jserv]

Depends on #47