std: Make unlink() more consistent

Currently when a read-only file has unlink() invoked on it on windows, the call
will fail. On unix, however, the call will succeed. In order to have a more
consistent behavior across platforms, this error is recognized on windows and
the file is changed to read-write before removal is attempted.
auto merge of #15227 : alexcrichton/rust/windows-unlink, r=brson

Currently when a read-only file has unlink() invoked on it on windows, the call
will fail. On unix, however, the call will succeed. In order to have a more
consistent behavior across platforms, this error is recognized on windows and
the file is changed to read-write before removal is attempted.