arch: arm: initialize mode variable upon user space enter

This commit initializes the thread.mode variable, right before
dropping thread privilege level to user mode. This is required,
as we need to know the privilege level of the thread, in case
we need to context-switch it -out and -in again (e.g. in case
an interrupt triggers a context-switch).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>