remove assumptions on a single libc

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on October 23, 2012 16:35:47_

I've incrementally been doing this in alloc code as I've hit problems from
real Windows scenarios.

xref:
- N.B.: this routine is not very reliable for two reasons: first, there can
- be multiple libc routines; second, the app can load a libc module after
- startup. We could switch to an interval tree that tracks load and unload
- events for pc_is_in_libc() and try to get rid of assumptions that there's
- just one libc.
*/
app_pc
get_libc_base(app_pc *libc_end_out OUT)

it will be most difficult for replace_malloc which needs to know which Heap
to use. we'll have to track who imports from whom.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1059_