target/riscv/cpu.c: add priv_spec validate/disable_exts helpers

target/riscv/cpu.c: add priv_spec validate/disable_exts helpers

We're doing env->priv_spec validation and assignment at the start of
riscv_cpu_realize(), which is fine, but then we're doing a force disable
on extensions that aren't compatible with the priv version.

This second step is being done too early. The disabled extensions might be
re-enabled again in riscv_cpu_validate_set_extensions() by accident. A
better place to put this code is at the end of
riscv_cpu_validate_set_extensions() after all the validations are
completed.

Add a new helper, riscv_cpu_disable_priv_spec_isa_exts(), to disable the
extesions after the validation is done. While we're at it, create a
riscv_cpu_validate_priv_spec() helper to host all env->priv_spec related
validation to unclog riscv_cpu_realize a bit.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230517135714.211809-8-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>