Avoid duplications in RISC-V exception handlers

[jserv]

Code duplication in the function body of the RISC-V exception handlers might be a maintenance headache. That is, we shall refine these functions in src/emulate.c:

• rv_except_insn_misaligned
• rv_except_load_misaligned
• rv_except_store_misaligned
• rv_except_illegal_insn
• rv_except_breakpoint proposed in Implement ebreak properly #60

Code generation using preprocessor macros may be an approach to avoid such duplications.