sched, sparc32: Update scheduler comments in copy_thread()

There's no finish_arch_switch() anymore in the latest scheduler tree.
Also update some other details.

Cc: David S. Miller <davem@davemloft.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: sparclinux@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>