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]