cpu/epiphany*: objdump: Don't print zero offset displacement for ldr*

Want

``` asm
ldrb ip,[ip]
```

Actual

``` asm
ldrb ip,[ip,+0x0]
```

We treat the sign bit as a separate field (so it's printer is not aware of context, i.e., cannot see offset value), otherwise it would have been trivial.

This is a harmless regression, it only affect disassembling.
