8197981: Missing return statement in __sync_val_compare_and_swap_8

Reviewed-by: andrew, dholmes

src/hotspot/os_cpu/bsd_zero/os_bsd_zero.cpp

@@ -435,6 +435,7 @@ extern "C" {
     long long unsigned int oldval,
     long long unsigned int newval) {
     ShouldNotCallThis();
+    return 0; // silence compiler warnings
   }
 };
 #endif // !_LP64

src/hotspot/os_cpu/linux_zero/os_linux_zero.cpp

@@ -479,7 +479,7 @@ extern "C" {
     long long unsigned int oldval,
     long long unsigned int newval) {
     ShouldNotCallThis();
-    return 0; // silence compiler compiler warnings
+    return 0; // silence compiler warnings
   }
 };
 #endif // !_LP64