Fix C++ build for Cygwin

Fix C++ build for Cygwin

gdb/ChangeLog:

2016-06-02  Jon Turney  <jon.turney@dronecode.org.uk>

	* windows-nat.c (handle_output_debug_string): Return type of
	gdb_signal_from_host() is gdb_signal, not an int.
	(windows_get_exec_module_filename): Add pointer casts for C++.

gdb/gdbserver/ChangeLog:

2016-06-02  Jon Turney  <jon.turney@dronecode.org.uk>

	* win32-low.c (win32_create_inferior): Add pointer casts for C++.