Generalize IR (instr_is_syscall() in particular) for offline/standalone cross-OS use

For use cases where decoding is done under a different OS than recording (e.g., with drmemtrace you could record on Windows and run raw2trace or an analyzer on Linux) we would like the decoder/encoder/IR to be cross-OS. instr_is_syscall() is one particular case where for OP_int it has OS-specific behavior. We'd have to figure out whether to always relax the OS defines inside the x86 instr_is_syscall() or only do that when in standalone mode or have a separate instr_is_maybe_syscall(). We also need to look over the rest of the IR and IR API to see if there are any other cases like this.