Driver: adjust library search path for -sdk

Adjust the library search path so that the normal build of the
runtime/sdk overlay can be used as the SDK content.  This allows for the
rest of the search to be cascading.

Author: compnerd

lib/Driver/ToolChains.cpp

@@ -1195,6 +1195,9 @@ void ToolChain::getRuntimeLibraryPaths(SmallVectorImpl<std::string> &runtimeLibP
     scratchPath = SDKPath;
     llvm::sys::path::append(scratchPath, "usr", "lib", "swift");
     runtimeLibPaths.push_back(scratchPath.str());
+
+    llvm::sys::path::append(scratchPath, getPlatformNameForTriple(getTriple()));
+    runtimeLibPaths.push_back(scratchPath.str());
   }
 }