"Fix" TestFileHandle.py on non-darwin platforms

This test exposed a very long standing issue that the python file
objects returned by the FILE* typemap were unusable on non-darwin
platforms. The reason they work on darwin is that they rely on a
non-standard extension to fetch the "mode" of a FILE* object. On other
platforms, this code was #ifdefed out, and so we were returning an empty
mode.

As there's no portable way to get this information, I just change the
non-darwin path to return "r+", which should permit both reading and
writing operations on the object. If the underlying file descriptor
turns out to be incompatible with this mode, the operating system should
return EBADF (or equivalent), instead of the "file not open for XXX"
error from python.

llvm-svn: 373573