Consolidate Linux openat implementation

This patch consolidates the open Linux syscall implementation on
sysdeps/unix/sysv/linux/open{64}.c.  The changes are:

  1. Remove wordsize-64 openat{64}.
  2. For architetures that define __OFF_T_MATCHES_OFF64_T openat64
     will be default one with alias to required symbols.  Otherwise
     openat64 will pass the required O_LARGEFILE flag on syscall.

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

	* sysdeps/unix/sysv/linux/openat.c (__libc_openat): Build only
	for !__OFF_T_MATCHES_OFF64_T.
	* sysdeps/unix/sysv/linux/openat64.c (__libc_openat64): New
	implementation based on open64.
	* sysdeps/unix/sysv/linux/wordsize-64/openat.c: Remove file.
	* sysdeps/unix/sysv/linux/wordsize-64/openat64.c: Likewise.