Make unknown opcodes more visible: add drmemtrace invariant check on unknown opcodes; add rstat?

This is in response to https://github.com/DynamoRIO/dynamorio/issues/5744#issuecomment-1331472832 where having LDAPR as an unknown opcode caused crashes (exact culprit still unknown as the xx operands should have been sufficient), inaccurate traces, and problems later with traces containing xx read by tools updated to know it was LDAPR.

There are two proposals here for AArch64 OP_xx opcodes:

1. Add a drmemtrace invariant check on these opcodes so that we catch any missing opcodes in traces
2. Add a release-build statistic on the OP_xx count so we can easily see in an rstat summary whether any were present after a run