linux-user: fix clock_nanosleep()

If the call is interrupted by a signal handler, it fails with error EINTR
and if "remain" is not NULL and "flags" is not TIMER_ABSTIME, it returns
the remaining unslept time in "remain".

Update linux-user to not overwrite the "remain" structure if there is no
error.

Found with "make check-tcg", linux-test fails on nanosleep test:

  TEST    linux-test on x86_64
.../tests/tcg/multiarch/linux-test.c:242: nanosleep

Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/syscall.c

@@ -11831,8 +11831,14 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
         target_to_host_timespec(&ts, arg3);
         ret = get_errno(safe_clock_nanosleep(arg1, arg2,
                                              &ts, arg4 ? &ts : NULL));
-        if (arg4)
+        /*
+         * if the call is interrupted by a signal handler, it fails
+         * with error -TARGET_EINTR and if arg4 is not NULL and arg2 is not
+         * TIMER_ABSTIME, it returns the remaining unslept time in arg4.
+         */
+        if (ret == -TARGET_EINTR && arg4 && arg2 != TIMER_ABSTIME) {
             host_to_target_timespec(arg4, &ts);
+        }
 
 #if defined(TARGET_PPC)
         /* clock_nanosleep is odd in that it returns positive errno values.