cygwin: workaround GCC 6 changes

GCC 6 includes a number of new warnings which cause Cygwin to either not
compile, or not work properly even if said warnings are ignored:

https://cygwin.com/ml/cygwin-developers/2017-09/msg00000.html
https://gcc.gnu.org/gcc-6/porting_to.html

For now, we use the flags necessary to revert to GCC 5 behaviour until we
can fix the code properly.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

winsup/cygwin/crt0.c

@@ -20,7 +20,9 @@ void
 mainCRTStartup ()
 {
 #ifdef __i386__
+#pragma GCC diagnostic ignored "-Wframe-address"
   (void)__builtin_return_address(1);
+#pragma GCC diagnostic pop
   asm volatile ("andl $-16,%%esp" ::: "%esp");
 #endif