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

Swift SVN r23202

stdlib/core/String.swift

@@ -266,6 +266,7 @@ extension String {
   }
 }
 
+#if _runtime(_ObjC)
 /// Compare two strings using the Unicode collation algorithm in the
 /// deterministic comparison mode.  (The strings which are equivalent according
 /// to their NFD form are considered equal.  Strings which are equivalent
@@ -284,23 +285,36 @@ extension String {
 public func _stdlib_compareNSStringDeterministicUnicodeCollation(
   lhs: AnyObject, rhs: AnyObject
 )-> Int32
+#endif
 
 extension String: Equatable {
 }
 
 public func ==(lhs: String, rhs: String) -> Bool {
+#if _runtime(_ObjC)
   // Note: this operation should be consistent with equality comparison of
   // Character.
   return _stdlib_compareNSStringDeterministicUnicodeCollation(
     lhs._bridgeToObjectiveCImpl(), rhs._bridgeToObjectiveCImpl()) == 0
+#else
+  // FIXME: Actually implement. For now, all strings are unequal.
+  // rdar://problem/18878343
+  return false
+#endif
 }
 
 extension String : Comparable {
 }
 
 public func <(lhs: String, rhs: String) -> Bool {
+#if _runtime(_ObjC)
   return _stdlib_compareNSStringDeterministicUnicodeCollation(
     lhs._bridgeToObjectiveCImpl(), rhs._bridgeToObjectiveCImpl()) < 0
+#else
+  // FIXME: Actually implement. For now, all strings are unequal
+  // rdar://problem/18878343
+  return false
+#endif
 }
 
 // Support for copy-on-write
@@ -327,8 +341,10 @@ extension String {
   }
 }
 
+#if _runtime(_ObjC)
 @asmname("swift_stdlib_NSStringNFDHashValue")
 func _stdlib_NSStringNFDHashValue(str: AnyObject) -> Int
+#endif
 
 extension String : Hashable {
   /// The hash value.
@@ -346,11 +362,17 @@ extension String : Hashable {
 #else
     let hashOffset = 0x429b126688ddcc21
 #endif
+#if _runtime(_ObjC)
     // FIXME(performance): constructing a temporary NSString is extremely
     // wasteful and inefficient.
     let cocoaString =
       unsafeBitCast(self._bridgeToObjectiveCImpl(), _NSStringCoreType.self)
     return hashOffset ^ _stdlib_NSStringNFDHashValue(cocoaString)
+#else
+    // FIXME: Actually implement. For now, all strings have the same hash.
+    // rdar://problem/18878343
+    return hashOffset
+#endif
   }
 }
 
@@ -392,10 +414,6 @@ public func += (inout lhs: String, rhs: String) {
   }
 }
 
-// Comparison operators
-extension String : Comparable {
-}
-
 extension String {
   /// Low-level construction interface used by introspection
   /// implementation in the runtime library.  Constructs a String in