target/riscv: add 'rva22u64' CPU

target/riscv: add 'rva22u64' CPU

This CPU was suggested by Alistair [1] and others during the profile
design discussions. It consists of the bare 'rv64i' CPU with rva22u64
enabled by default, like an alias of '-cpu rv64i,rva22u64=true'.

Users now have an even easier way of consuming this user-mode profile by
doing '-cpu rva22u64'. Extensions can be enabled/disabled at will on top
of it.

We can boot Linux with this "user-mode" CPU by doing:

-cpu rva22u64,sv39=true,s=true,zifencei=true

[1] https://lore.kernel.org/qemu-riscv/CAKmqyKP7xzZ9Sx=-Lbx2Ob0qCfB7Z+JO944FQ2TQ+49mqo0q_Q@mail.gmail.com/

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231218125334.37184-19-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>