support 32-bit ELF in 64-bit drsyms (and possibly vice versa)

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on August 29, 2014 15:48:41_

Xref issue #1345 for the core supporting mixed libraries.

drsyms, though, has a greater need for it, for offline tools that want to analyze a mixture (e.g., drcov analyzing several runs of different bitwidths but from the same sources).

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1532_

Start by reading the drsyms implementation and the mixed-library context in issue #1345. Check how drcov and other offline tools analyze ELF inputs of different bitwidths. Done means a 64-bit drsyms can analyze 32-bit ELF files, and the reverse case is supported if applicable, with coverage for mixed inputs.

Written by the indexing model from the issue text.