target/riscv: move riscv_cpu_validate_set_extensions() to tcg-cpu.c

This function is the core of the RISC-V validations for TCG CPUs, and it
has a lot going on.

Functions in cpu.c were made public to allow them to be used by the KVM
accelerator class later on. 'cpu_cfg_ext_get_min_version()' is notably
hard to move it to another file due to its dependency with isa_edata_arr[]
array, thus make it public and use it as is for now.

riscv_cpu_validate_set_extensions() is kept public because it's used by
csr.c in write_misa().

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: <20230925175709.35696-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>