linux-user: Fix waitid return of siginfo_t and rusage

linux-user: Fix waitid return of siginfo_t and rusage

The copy back to siginfo_t should be conditional only on arg3,
not the specific values that might have been written.
The copy back to rusage was missing entirely.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2262
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alex Fan <alex.fan.q@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit f0907ff4cae743f1a4ef3d0a55a047029eed06ff)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>