target/riscv: Allow setting ISA extensions via CPU props

target/riscv: Allow setting ISA extensions via CPU props

This patch allows us to enable/disable the RISC-V ISA extensions from
the QEMU command line. This works with the rv32 and rv64 machines. The
idea is that in the future we can now add extensions and leave them
disabled by default until enabled by the user.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>