target/riscv: Add itrigger support when icount is enabled

target/riscv: Add itrigger support when icount is enabled

The max count in itrigger can be 0x3FFF, which will cause a no trivial
translation and execution overload.

When icount is enabled, QEMU provides API that can fetch guest
instruction number. Thus, we can set an timer for itrigger with
the count as deadline.

Only when timer expires or priviledge mode changes, do lazy update
to count.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221013062946.7530-3-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>