Cygwin: suppress a warning generated with w32api >= 12.0.0

w32api 12.0.0 adds the returns_twice attribute to RtlCaptureContext().
There's some data-flow interaction with using it inside a while loop
which causes a maybe-uninitialized warning.

../../../../winsup/cygwin/exceptions.cc: In member function 'int _cygtls::call_signal_handler()':
../../../../winsup/cygwin/exceptions.cc:1720:33: error: '<anonymous>' may be used uninitialized in this function [-Werror=maybe-uninitialized]

(cherry picked from commit 7e3c833)

winsup/cygwin/exceptions.cc

@@ -1717,7 +1717,10 @@ _cygtls::call_signal_handler ()
 		 context, unwind to the caller and in case we're called
 		 from sigdelayed, fix the instruction pointer accordingly. */
 	      context.uc_mcontext.ctxflags = CONTEXT_FULL;
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
 	      RtlCaptureContext ((PCONTEXT) &context.uc_mcontext);
+#pragma GCC diagnostic pop
 	      __unwind_single_frame ((PCONTEXT) &context.uc_mcontext);
 	      if (stackptr > stack)
 		{