target/riscv/cpu.c: create flag for ziccrse

target/riscv/cpu.c: create flag for ziccrse

At this moment ziccrse is a TCG always enabled named feature for
priv_ver > 1.11 that has no exclusive flag. In the next patch we'll make
the KVM driver turn ziccrse off if the extension isn't available in the
host, and we'll need an ext_ziccrse flag in the CPU state for that.

Create an exclusive flag for it like we do with other named features.
As with any named features we already have, it won't be exposed to
users.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20250221153758.652078-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>