Add breakpoint on instruction level

[stefanberndtsson]

Make it possible to put a breakpoint on the base instruction instead of the explicit opcode (which is essentially the only possible way today, via watchpoints), so that one can break on any ROXL for example, regardless of its parameters.

This can (mostly) be done by using the actual function pointer value as the breakpoint trigger, because it's usually a generic dispatcher for the entire instruction handler.