kernel/arm: add comment about _is_next_thread_current

Normally, _is_next_thread_current() must be called with interrupts
locked, but the ARM interrupt exit code does not have to do that. Add
explanation why.

Change-Id: Id383b47a055fdd6fbd5afffa52772e92febde98f
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>