target/riscv: Fix bug in getting trap cause name for trace_riscv_trap

target/riscv: Fix bug in getting trap cause name for trace_riscv_trap

When the cause number is equal to or greater than 23, print "(unknown)" in
trace_riscv_trap. The max valid number of riscv_excp_names is 23, so the last
excpetion "guest_store_page_fault" can not be printed.

In addition, the current check of cause is invalid for riscv_intr_names. So
introduce riscv_cpu_get_trap_name to get the trap cause name.

Signed-off-by: Yifei Jiang <jiangyifei@huawei.com>
Signed-off-by: Yipeng Yin <yinyipeng1@huawei.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20200814035819.1214-1-jiangyifei@huawei.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>