RFR: 8145553: Fix warnings in AArch64 directory

[Andrew Haley]

http://cr.openjdk.java.net/~aph/8145553/

These are mostly because ShouldNotReachHere() is not marked as
noreturn.  I understand that this has been discussed in the past and
it was decided not to fix it because it broke stack traces.

I've marked all of the bogus assignments as unreachable so that they
can be removed if ever ShouldNotReachHere() is fixed.

Andrew.