[runtime] Add implementation of _swift_strlcpy

This safer function is missing on Linux.

stdlib/public/runtime/Portability.cpp

@@ -0,0 +1,29 @@
+//===--- Portability.cpp - -----------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===---------------------------------------------------------------------===//
+//
+// Implementations of the stub APIs that make portable runtime easier to write.
+//
+//===---------------------------------------------------------------------===//
+
+#include "swift/Runtime/Portability.h"
+#include <cstring>
+
+size_t _swift_strlcpy(char *dst, const char *src, size_t maxlen) {
+  const size_t srclen = std::strlen(src);
+  if (srclen < maxlen) {
+    std::memmove(dst, src, srclen + 1);
+  } else if (maxlen != 0) {
+    std::memmove(dst, src, maxlen - 1);
+    dst[maxlen - 1] = '\0';
+  }
+  return srclen;
+}