target/riscv: Add a property to set vl to ceil(AVL/2)

target/riscv: Add a property to set vl to ceil(AVL/2)

RVV spec allows implementations to set vl with values within
[ceil(AVL/2),VLMAX] when VLMAX < AVL < 2*VLMAX. This commit adds a
property "rvv_vl_half_avl" to enable setting vl = ceil(AVL/2). This
behavior helps identify compiler issues and bugs.

Signed-off-by: Jason Chien <jason.chien@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-ID: <20240722175004.23666-1-jason.chien@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>