Add needed functions to glibc-compat for pkg

ports/glibc-compat/src/writev.c

+/*
+ * Copyright 2015 The Native Client Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#include <unistd.h>
+#include <sys/uio.h>
+#include <errno.h>
+
+int writev(int fd, const struct iovec *iov, int iovcnt) {
+  int i = 0, ret = 0, n = 0, len = 0;
+  char* base;
+  errno = 0;
+  while (i < iovcnt) {
+    len = iov->iov_len;
+    base = iov->iov_base;
+    while (len > 0) {
+      ret = write(fd, base, len);
+      if (ret < 0 && n == 0) return -1;
+      if (ret <= 0) return n;
+      errno = 0;
+      n += ret;
+      len -= ret;
+      base += ret;
+    }
+    iov++;
+    i++;
+  }
+  return n;
+}
+
+ssize_t readv(int fd, const struct iovec *iov, int iovcnt) {
+  int i = 0, ret = 0, n = 0, len = 0;
+  char* base;
+  errno = 0;
+  while (i < iovcnt) {
+    len = iov->iov_len;
+    base = iov->iov_base;
+    while (len > 0) {
+      ret = read(fd, base, len);
+      if (ret < 0 && n == 0) return -1;
+      if (ret <= 0) return n;
+      errno = 0;
+      n += ret;
+      len -= ret;
+      base += ret;
+    }
+    iov++;
+    i++;
+  }
+  return n;
+}