support binary offset annotations for custom allocators in third-party modules

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on November 15, 2013 11:48:27_

For third-party components in applications being tested, it's not possible
to use the typical source annotations to mark custom allocators. This
issue proposes adding some way to specify the module offsets of malloc,
free, etc. Xref the old -libc_offsets option.

Note that we could possibly hack the symcache file to accomplish this.

We may want an option to wrap instead of replacing custom allocators, with
no redzone, for cases where there are assumptions or dependences on
particular properties of the custom allocator, or where there auxiliary
routines we don't have support for (or haven't added options for: size
query, etc.).

We might also want a Dr. Memory mode that tries to identify custom
allocators by monitoring live mmap regions and watching for function return
values that are always in an mmap range or something.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1384_