[libc++][z/OS] Fixup two linear_congruential_engine tests (#92261)

Author: zibi2

libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/assign.pass.cpp

@@ -83,7 +83,10 @@ int main(int, char**)
     test<unsigned int>();
     test_ext<unsigned int>();
     test<unsigned long>();
+    // This isn't implemented on platforms without __int128
+#ifndef _LIBCPP_HAS_NO_INT128
     test_ext<unsigned long>();
+#endif
     test<unsigned long long>();
     // This isn't implemented on platforms without __int128
 #ifndef _LIBCPP_HAS_NO_INT128

libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/copy.pass.cpp

@@ -82,7 +82,10 @@ int main(int, char**)
     test<unsigned int>();
     test_ext<unsigned int>();
     test<unsigned long>();
+    // This isn't implemented on platforms without __int128
+#ifndef _LIBCPP_HAS_NO_INT128
     test_ext<unsigned long>();
+#endif
     test<unsigned long long>();
     // This isn't implemented on platforms without __int128
 #ifndef _LIBCPP_HAS_NO_INT128