target/riscv: fix exception index on instruction access fault

target/riscv: fix exception index on instruction access fault

When no MMU is used and the guest code attempts to fetch an instruction
from an invalid memory location, the exception index defaults to a data
load access fault, rather an instruction access fault.

Signed-off-by: Emmanuel Blot <emmanuel.blot@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: FB9EA197-B018-4879-AB0F-922C2047A08B@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>