target/riscv: rvv: Add tail agnostic for vector mask instructions

target/riscv: rvv: Add tail agnostic for vector mask instructions

The tail elements in the destination mask register are updated under
a tail-agnostic policy.

Signed-off-by: eop Chen <eop.chen@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <165449614532.19704.7000832880482980398-14@git.sr.ht>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>