Use wide dbghelp routines in drsyms when passing paths

This should make it possible to load syms for a file with a non-ASCII path, but
I haven't been able to construct a satisfying test.

Review URL: https://codereview.appspot.com/7388044

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1928 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6