add variant of opnd_create_relative_addr() that takes in an unchanging offset

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on October 03, 2012 15:15:08_

opnd_create_relative_addr() takes in an absolute addr that is relatived
when encoded. I want to pass a relative address. my use case is embedding
a target in the instruction stream and I know it's right after my jmp\* and
thus the relative offset is 0. alternatively, adding support for instr
targets as rip-relative data addresses would work for that case ( issue #222 ) but maybe there are other scenarios where we'd want an offset.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=929_