Bug 445415 - arm64 front end: alignment checks missing for atomic instructions.

Bug 445415 - arm64 front end: alignment checks missing for atomic instructions.

For the arm64 front end, none of the atomic instructions have address
alignment checks included in their IR.  They all should.  The effect of
missing alignment checks in the IR is that, since this IR will in most cases
be translated back to atomic instructions in the back end, we will get
alignment traps (SIGBUS) on the host side and not on the guest side, which is
(very) incorrect behaviour of the simulation.