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>

linux-user/riscv/signal.c

@@ -40,8 +40,9 @@ struct target_ucontext {
     unsigned long uc_flags;
     struct target_ucontext *uc_link;
     target_stack_t uc_stack;
-    struct target_sigcontext uc_mcontext;
     target_sigset_t uc_sigmask;
+    uint8_t   __unused[1024 / 8 - sizeof(target_sigset_t)];
+    struct target_sigcontext uc_mcontext QEMU_ALIGNED(16);
 };
 
 struct target_rt_sigframe {