Integrate GenSimpoint refcpu for difftest and oracle

[Zhu-Liyan]

Summary

• integrate GenSimpoint librefcpu.a into the OoO simulator
• replace direct use of in-tree RefCpu with refcpu API calls
• keep separate refcpu instances for difftest and frontend oracle
• align refcpu ebreak PC behavior with the old RefCpu implementation

Main Changes

• top-level Makefile links libs/librefcpu.a
• difftest/oracle paths now use the refcpu API instead of direct RefCpu
  internals
• update diff/diff.cpp, diff/br_oracle.cpp, diff/include/diff.h, and
  main.cpp
• update submodule axi-interconnect-kit with a small toolchain build fix