csr: use "keep old value" strategy to implement WARL of xtvec.mode

[huxuan0307]

• This commit modify the strategy of WARL of xtvec.mode
• At present, only spike uses write mask to handle the illegal value of xtvec.mode. In this strategy, when write mtvec.mode with illegal value 0b11, spike will always store legal value 0b01(Vectored) in mtvec.mode.
• sail-riscv and qemu uses "keep old value" strategy to handle the illegal write value. When write mtvec.mode with illegal value 0b11 or 0b10, sail-riscv and qemu will always keep the old value in mtvec.mode which must be legal value.