Fix unwind_reg_info tool.

I was using the pc as the offest into the elf. That is obviously not
correct. Added an optional OFFSET argument like in unwind_info along
with this change.

Test: Verified that with no offest works, verified with a zero offset
Test: works, verified with a non-zero offset results in a bad elf
Test: on an elf without an offset.
Change-Id: I4b6d02609627288e9f8a0eb26988d03adf95cb1f