Python 3 - Don't add the _d suffix to the symlink on Windows.

In Python 2, a debug extension module required an _d suffix, so
for example the extension module `_lldb` would be backed by the file
`_lldb_d.pyd` if built in debug mode, and `_lldb.pyd` if built in
release mode.  In Python 2, although undocumented, this seems to
no longer be the case, and even for a debug extension module, the
interpreter will only look for the `_lldb.pyd` name.

llvm-svn: 252026