linux-user/riscv: fix up struct target_ucontext definition

linux-user/riscv: fix up struct target_ucontext definition

As struct target_ucontext will be transfered to signal handler, it
must keep pace with struct ucontext_t defined in Linux kernel.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200412020830.607-1-zhiwei_liu@c-sky.com
Message-Id: <20200412020830.607-1-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>