target/riscv: rework 'vext_spec'

target/riscv: rework 'vext_spec'

The same rework did in 'priv_spec' is done for 'vext_spec'. This time is
simpler, since we only accept one value ("v1.0") and we'll always have
env->vext_ver set to VEXT_VERSION_1_00_0, thus we don't need helpers to
convert string to 'vext_ver' back and forth like we needed for
'priv_spec'.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Vladimir Isaev <vladimir.isaev@syntacore.com>
Message-ID: <20240105230546.265053-8-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>