arch: arm: Optimise Cortex-R exception return function.

z_arm_exc_exit (z_arm_int_exit) requires the current execution mode to
be specified as a parameter (through r0). This is not necessary because
this value can be directly read from CPSR.

This commit modifies the exception return function to retrieve the
current execution mode from CPSR and removes all provisions for passing
the execution mode parameter.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>