Extend drcpusim to simulate non-existence for AArch64

Today drcpusim supports a set of legacy x86 processors and identifies opcodes present in an application's dynamic code stream which are not supported on a given processor.

This is a feature request to extend that functionality to AArch64 ISA variants. There are plans to add AArch64 ISA version information directly into DR's decoder, which should make this straightforward to implement (as opposed to the current x86 implementation which relies on checking sets of opcodes as the DR x86 decoder does not have information availability of each opcode).