target/riscv: implementation-defined constant parameters

target/riscv: implementation-defined constant parameters

vlen is the vector register length in bits.
elen is the max element size in bits.
vext_spec is the vector specification version, default value is v0.7.1.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200701152549.1218-3-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>