tools/memory-model: Tie acquire loads to reads-from

tools/memory-model: Tie acquire loads to reads-from

This commit explicitly makes the connection between acquire loads and
the reads-from relation.  It also adds an entry for happens-before,
and refers to the corresponding section of explanation.txt.

Reported-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>