target/riscv: Refactor some of the generic vector functionality

Take some functions/macros out of `vector_helper` and put them in a new
module called `vector_internals`. This ensures they can be used by both
vector and vector-crypto helpers (latter implemented in proceeding
commits).

Signed-off-by: Kiran Ostrolenk <kiran.ostrolenk@codethink.co.uk>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Max Chou <max.chou@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230711165917.2629866-2-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>