Compilation Failure When Using clang and Enabling RVB & PERF_OPT

[cebarobot]

clang is not happy with current RVB implementation when perfomance optimization, because:

• NEMU defines some functions in src/isa/riscv64/instr/rvb/rvintrin.h.
• rvintrin.h is included in file src/isa/riscv64/instr/rvb/exec.h.
• rvb/exec.h is included in file src/isa/riscv64/include/isa-exec.h.
• isa-exec.h is included in function execute() in file src/cpu/cpu-exec.c, when perfomance optimization is enabled.

So that there is some functions are defined in another function, which clang complained about.

Also, rvintrin.h is really weird. It has another copyright header. I think this file is from here, but the two files are very different.

I think we should rewrite the implementation of RVB.