target/riscv: rvv: Modify minimum VLEN according to enabled vector extensions

target/riscv: rvv: Modify minimum VLEN according to enabled vector extensions

According to the RISC-V unprivileged specification, the VLEN should be greater
or equal to the ELEN. This commit modifies the minimum VLEN based on the vector
extensions and introduces a check rule for VLEN and ELEN.

  Extension     Minimum VLEN
* V                      128
* Zve64[d|f|x]            64
* Zve32[f|x]              32

Signed-off-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250923090729.1887406-3-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>