target/riscv: rvv-1.0: add evl parameter to vext_ldst_us()

Add supports of Vector unit-stride mask load/store instructions
(vlm.v, vsm.v), which has:
    evl (effective vector length) = ceil(env->vl / 8).

The new instructions operate the same as unmasked byte loads and stores.
Add evl parameter to reuse vext_ldst_us().

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-74-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>