arch: arm: cortex_m: add ip & lr to the clobber list

Calls to other function may clobber ip & lr too so these register need to
be added to the clobberlist.
r3 is not actually used in z_arm_switch_to_main_no_multithreading so it is
also removed from the clobber list.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>