Consolidate Linux write syscall

This patch consolidates the write Linux syscall implementation on
sysdeps/unix/sysv/linux/write.c.

Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
arch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.

	* include/unistd.h (write): Add hidden proto.
	* io/Makefile (CFLAGS-write.c): New rule.
	* nptl/Makefile (CFLAGS-write.c): Likewise.
	* sysdeps/unix/sysv/linux/write.c: New file.

sysdeps/unix/sysv/linux/write.c

@@ -0,0 +1,33 @@
+/* Linux write syscall implementation.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+#include <sysdep-cancel.h>
+
+/* Write NBYTES of BUF to FD.  Return the number written, or -1.  */
+ssize_t
+__libc_write (int fd, const void *buf, size_t nbytes)
+{
+  return SYSCALL_CANCEL (write, fd, buf, nbytes);
+}
+libc_hidden_def (__libc_write)
+
+weak_alias (__libc_write, __write)
+libc_hidden_weak (__write)
+weak_alias (__libc_write, write)
+libc_hidden_weak (write)