linux-user: Add default-configs for mipsn32[el]

Prepares for mipsn32[el]-linux-user targets.
Signed-off-by: NUlricht Hecht <uli@suse.de>
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

default-configs/mipsn32-linux-user.mak

+# Default configuration for mipsn32-linux-user

default-configs/mipsn32el-linux-user.mak

+# Default configuration for mipsn32el-linux-user