linux-user: Fix getresuid, getresgid if !USE_UID16

linux-user: Fix getresuid, getresgid if !USE_UID16

The size of the UID/GID types depends on whether USE_UID16 is
defined. Define a new put_user_id() which writes a uid/gid
type to guest memory. This fixes getresuid and getresgid, which
were always storing 16 bits even if the uid type was 32 bits.

Reported-by: Michael Matz <matz@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>