target/riscv: rvv: Add option 'rvv_ta_all_1s' to enable optional tail agnostic behavior

target/riscv: rvv: Add option 'rvv_ta_all_1s' to enable optional tail agnostic behavior

According to v-spec, tail agnostic behavior can be either kept as
undisturbed or set elements' bits to all 1s. To distinguish the
difference of tail policies, QEMU should be able to simulate the tail
agnostic behavior as "set tail elements' bits to all 1s".

There are multiple possibility for agnostic elements according to
v-spec. The main intent of this patch-set tries to add option that
can distinguish between tail policies. Setting agnostic elements to
all 1s allows QEMU to express this.

This commit adds option 'rvv_ta_all_1s' is added to enable the
behavior, it is default as disabled.

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>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <165449614532.19704.7000832880482980398-16@git.sr.ht>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>