Bring up RegOpenKeyExW stub

This works around the Get-ExecutionPolicy Pester test failure. It's not a good solution, but it's also (seemingly) not our code that is calling the extern function. This needs to be fixed in CoreFX itself.

src/registry-stub/api-ms-win-core-registry-l1-1-0.c

@@ -1,5 +1,10 @@
 #include <stdint.h>
 
+int32_t RegOpenKeyExW(void* hKey,uint16_t* lpSubKey,uint32_t ulOptions,uint32_t samDesired,void* phkResult)
+{
+    return 1;
+}
+
 int32_t RegCloseKey(void* handle)
 {
     return 0;