RISC-V Vectored-Mode Trap Entry Support?

[Coekjan]

https://github.com/rcore-os/trapframe-rs/blob/0f27d58f5a13ae08cf5293ab0999cd12878950e1/src/arch/riscv/trap.rs#L44

This line just sets Direct-Mode and trap.S does not support Vectored-Mode now. I suggest that this library support Vectored-Mode on RISC-V.

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这行直接设置为了 Direct-Mode，而且 trap.S 目前也并不支持 Vectored-Mode。我建议本库支持 RISC-V 上的 Vectored-Mode。