linux-user: Implement PR_GET_TID_ADDRESS

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 4871c4b..e12d969 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -6468,8 +6468,10 @@
         }
 
     case PR_GET_TID_ADDRESS:
-        /* TODO */
-        return -TARGET_EINVAL;
+        {
+            TaskState *ts = env_cpu(env)->opaque;
+            return put_user_ual(ts->child_tidptr, arg2);
+        }
 
     case PR_GET_FPEXC:
     case PR_SET_FPEXC: