[llvm-objdump] Support nonzero section addresses in addSymbolizer

The previous calculations seem to have assumed that the section address would be zero.
This is true for relocatable object files, but certainly not for linked files like shared libraries.

Fixed the calculations to make them identical to the "real" `getInstruction` call below & added a regression test.

Reviewed By: scott.linder, simon_tatham

Differential Revision: https://reviews.llvm.org/D135430