arch: arm: update arch.mode in user thread system calls

This commit updates the thread.mode variable, in system
calls, whenever we modify the execution privilege level.
We need to do this, in order to be able to properly handle
context-switching triggered by ISRs, while doing the system
calls. The commit ensures that a context-switch, triggered
by an ISR during or after,a system call will preserve the
right privilege level.

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