[corlib] Use Directory.GetFiles/GetDirectories implementation from referencesource...

[corlib] Use Directory.GetFiles/GetDirectories implementation from referencesource (#4042)

We already had implementations of the Windows APIs like FindFirstFile/FindNextFile
in native code but we wrapped it with some convenience functions for our implementation.

The referencesource code works directly against those APIs so I added new icall functions
that are just used for passing data to/from native code.

Note that we have the same problem in the DirectoryInfo.GetFiles() implementation.
Right now that one still uses the old Mono implementation, switching it over to
the referencesource implementation will be done in a separate commit.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=42715