PC-relative operand support on ARM broken

For the ARM port (#1551 (closed)) we half-added PC-relative operand support, changing the headers to say that opnd_create_rel_addr() works on ARM, when in fact it will fail to encode. Plus, we never made opnd_is_rel_addr() supported on ARM in the headers or implementation.

We should either change the headers to not support REL_ADDR_kind at all, or we should implement full support via the query routines and in the encoder. The encoder already handles opnd_is_mem_instr() and rel addr opnd support is very, very similar, so I would vote for adding it.

Note that trying to auto-magically take a tool opnd marked as base-disp that has a PC base and re-relativize it is a whole other kettle of fish as that requires register spilling and is covered by #1834.