Better handling of realpath() failure in windows_make_so() on Cygwin

It seems Cygwin's realpath() can fail on certain DLLs (apparently some
AV software prevent it working on it's DLLs; See [1], [2]).  Warn rather
than stopping with an error if that occurs.

Based on an original patch from Tim Chick.

[1] https://cygwin.com/ml/cygwin/2014-08/msg00401.html
[2] https://cygwin.com/ml/cygwin/2015-11/msg00353.html

gdb/ChangeLog:

2016-01-20  Jon Turney  <jon.turney@dronecode.org.uk>

	* windows-nat.c (windows_make_so): Warn rather than stopping with
	an error if realpath() fails.