target/riscv: Enable xtheadsync under user mode

target/riscv: Enable xtheadsync under user mode

According to xtheadsync[1][2] documentation, it can be used in user mode and
the behavior is same with other priviledges.

[1]:https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadsync/sync.adoc
[2]:https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadsync/sync_i.adoc

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240204055228.900-1-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>