tools/memory-order: Improve key for SELF and SV

The key for "SELF" was missing completely and the key for "SV" was
a bit obtuse.  This commit therefore adds a key for "SELF" and improves
the one for "SV".

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: akiyks@gmail.com
Cc: boqun.feng@gmail.com
Cc: dhowells@redhat.com
Cc: j.alglave@ucl.ac.uk
Cc: linux-arch@vger.kernel.org
Cc: luc.maranget@inria.fr
Cc: npiggin@gmail.com
Cc: parri.andrea@gmail.com
Link: http://lkml.kernel.org/r/1526340837-12222-5-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/memory-model/Documentation/cheatsheet.txt

@@ -26,4 +26,5 @@ Key:	C:	Ordering is cumulative
 	DR:	Dependent read (address dependency)
 	DW:	Dependent write (address, data, or control dependency)
 	RMW:	Atomic read-modify-write operation
-	SV	Same-variable access
+	SELF:	Orders self, as opposed to accesses before and/or after
+	SV:	Orders later accesses to the same variable