__int128 is not supported by Visual Studio

Make the test trivially pass on Windows when not using Cygwin.

buildspec-windows.yml

@@ -48,7 +48,6 @@ phases:
         Remove-Item cbmc\byte_update5 -Force -Recurse
         Remove-Item cbmc\byte_update6 -Force -Recurse
         Remove-Item cbmc\byte_update7 -Force -Recurse
-        Remove-Item cbmc\unsigned___int128 -Force -Recurse
         Remove-Item cbmc-library\pipe-01 -Force -Recurse
         Remove-Item cpp -Force -Recurse
         Remove-Item cbmc-cpp -Force -Recurse

regression/cbmc/unsigned___int128/main.c

@@ -1,3 +1,8 @@
+#ifndef __GNUC__
+void reduce()
+{
+}
+#else
 # include <stdint.h>
 
 typedef unsigned __int128 uint128_t;
@@ -109,3 +114,4 @@ void reduce(
           out[2] < p[1] || (out[2]==p[1] && (out[1]<p[2] || out[1]==p[2]
           && (out[0]<p[3]))))));
 }
+#endif