in light mode, only intercept app's malloc and not system library mallocs

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on December 16, 2011 11:03:48_

for light mode, we could focus on just app's malloc and not system library
malloc: i.e., try intercepting just outer layers to reduce overheads of
extra redzones and of interception: though most calls are in app anyway so
it's not clear how valuable this would be.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=721_