update genhtml to work with Windows paths

From bruen...@google.com on October 08, 2014 12:26:29

The lcov genhtml has some assumptions on paths being in a UNIX format (e.g., see shorten_prefix() in genthml). We could update the drcov docs to talk about this, and maybe modify our import of genhtml to make it work on Windows-style paths.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1552