kernel/arm: fix missing interrupt lock around _is_next_thread_current()

This reverts commit

	"kernel/arm: add comment about _is_next_thread_current"

and fixes the interrupt locking issue.

The comment would have been right if only reads were done the ready
queue, but that is not the case. It turns out that the comment was written
ignoring the fact that _is_next_thread_current() updates the next thread
cache when fetching the next thread.

Change-Id: I21c9230f85f4f87a6bbf14fd4a9eb7e19b59f8c5
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>