target/riscv: Organise the CPU properties

Organise the CPU properties so that standard extensions come first
then followed by experimental extensions.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: b6598570f60c5ee7f402be56d837bb44b289cc4d.1634531504.git.alistair.francis@wdc.com