target/riscv/cpu.c: mark extensions arrays as 'const'

target/riscv/cpu.c: mark extensions arrays as 'const'

We'll need to export these arrays to the accelerator classes in the next
patches. Mark them as 'const' now because they should not be modified at
runtime.

Note that 'riscv_cpu_options' will also be exported, but can't be marked
as 'const', because the properties are changed via
qdev_property_add_static().

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230925175709.35696-8-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>