Use config directives to carve out a 'portable' kernel stdlib.

Swift SVN r23202

stdlib/core/ArrayBuffer.swift

@@ -15,6 +15,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if _runtime(_ObjC)
 import SwiftShims
 
 final internal
@@ -207,7 +208,7 @@ extension _ArrayBuffer {
   ) {
     _arrayNonSliceInPlaceReplace(&self, subRange, newCount, newValues)
   }
-  
+
   func _typeCheck(subRange: Range<Int>) {
     if !_isClassOrObjCExistential(T.self) {
       return
@@ -476,4 +477,4 @@ extension _ArrayBuffer {
     return i.getCocoa()
   }
 }
-
+#endif