RV64: fix several bugs of H extention and performance optimization

[cebarobot]

This PR fix several bugs of H extention and performance optimization.

• Fix compile error when enabling "Enable debug features: instruction tracing and watchpoint" (CONFIG_DEBUG)
  • Duplicated definition of ebreak caused this problem
• Fix the issue of EBREAK instruction causing segmentation fault when enabling "Performance optimization" (CONFIG_PERF_OPT) NEMU在开启性能优化选项后，执行EBREAK指令时出现段错误 #132
  • EBREAK was missed during decoding Type-I instruction, causing pointer tnext of its Decode struct to be NULL.
• Fix executing incorrect VS-mode instructions in M-mode.
  • Some VS-mode instructions and M-mode instructions have the same PC address. NEMU did not flush TCACHE when an exception which changing virtualization mode occurs. Fix it.

这个合并请求修复了 H 扩展和性能优化相关的若干问题。

• 修复了启用“Enable debug features: instruction tracing and watchpoint”（CONFIG_DEBUG）时出现的编译错误
  • 有关 EBREAK 的重复定义造成了此问题
• 修复了启用性能优化选项“Performance optimization”（CONFIG_PERF_OPT）时调用 EBREAK 指令造成 NEMU 段错误的问题。 NEMU在开启性能优化选项后，执行EBREAK指令时出现段错误 #132
  • 译码时指定 I 类型指令时漏掉了 EBREAK，导致其 Decode 项中的 tnext 为空指针。
• 修复了在 M 态错误执行 VS 态指令的问题
  • 某些 M 态代码和 VS 态代码具有相同的地址。当发生更改虚拟化模式的异常时，NEMU 没有刷新 TCACHE。修复了这个问题。