elf: Implement _dl_write

The generic version is parallel to _dl_writev.  It cannot use
_dl_writev directly because the errno value needs to be obtained
under a lock.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/dl-write.c

@@ -0,0 +1,30 @@
+/* Implementation of the _dl_write function.  Linux version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sysdep.h>
+#include <unistd.h>
+#include <ldsodefs.h>
+
+ssize_t
+_dl_write (int fd, const void *buffer, size_t length)
+{
+  long int r = INTERNAL_SYSCALL_CALL (write, fd, buffer, length);
+  if (INTERNAL_SYSCALL_ERROR_P (r))
+    r = - INTERNAL_SYSCALL_ERRNO (r);
+  return r;
+}