[SystemZ] Rename mapping table fields

Rename Function->DispKey and PairType->DispSize.  I'd originally used
"Function" because I thought it might be useful for other InstMappings.
However, it turns out that having two very similar instructions with the
same Function makes it pretty useless for anything other than the displacement
size key.  Other InstMappings will want the key to be defined for only one
instruction in the pair.

No behavioural change intended.
ARM: relax the atomic release barrier to "dmb ishst" on Swift

Swift cores implement store barriers that are stronger than the ARM
specification but weaker than general barriers. They are, in fact, just about
enough to provide the ordering needed for atomic operations with release
semantics.

This patch makes use of that quirk.