Add a /bin symlink for convenience.

We already have /etc and /sbin. As the Android world moves towards / being
on the system partition, the circumstances under which a /bin symlink
won't work are reduced. This should already be usable most of the time.

Bug: http://b/63142920
Test: `adb shell /bin/date`
Change-Id: I81c2209ae808ced186d05fbe1d5417ce8dd93ea7