AArch64: undefined behaviour in HotSpot cpu-specific code

Running AArch64 HotSpot with GCC undefined behaviour sanitizer reveals many cases of UB, including null pointer dereferences. In some cases we'll just have to live with it and be careful to test the VM, but in many cases the UB is pointless and can be made properly defined without significant loss of performance.
This is an umbrella bug report to capture them all; there will be several sub-reports.