Consolidate scandir{at}{64} implementation

Consolidate scandir{at}{64} implementation

This patch consolidates scandir{at}{64} implementation on just
the default dirent/scandir{at}{64}{_r}.c ones.  It changes the logic
to follow the conventions used on other code consolidation:

  * scandir{at} is only built for _DIRENT_MATCHES_DIRENT64 being 0.

  * scandir{at}{64} is always built and aliased to getdents for ABIs
    that define _DIRENT_MATCHES_DIRENT64 to 1.

Also on Linux the compat symbol for old non-LFS dirent64 definition
requires a platform-specific scandir64.c.

Checked on aarch64-linux-gnu, x86_64-linux-gnu, i686-linux-gnu,
sparcv9-linux-gnu, sparc64-linux-gnu, powerpc-linux-gnu, and
powerpc64le-linux-gnu.

	* dirent/scandir-tail-common.c: New file.
	* dirent/scandir-tail.c: Use scandir-tail-common.c.
	(__scandir_tail): Build iff _DIRENT_MATCHES_DIRENT64 is not defined.
	* dirent/scandir.c: Use scandir-tail-common.c.
	* dirent/scandirat.c: Likewise.
	* dirent/scandir64-tail.c: Use scandir-tail-common.c.
	* dirent/scandir64.c (scandir64): Always build and alias to scandir
	if _DIRENT_MATCHES_DIRENT64 is defined.
	* dirent/scandirat64.c (scandirat64): Likewise.
	* include/dirent.h (__scandir_tail): Only define iff
	_DIRENT_MATCHES_DIRENT64 is not defined.
	(__scandir64_tail): Define regardless.
	(__scandirat, scandirat64): Remove libc_hidden_proto.
	* sysdeps/unix/sysv/linux/arm/scandir64.c: Remove file.
	* sysdeps/unix/sysv/linux/m68k/scandir64.c: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/scandir64.c: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-32/scandir64.c: Likewise.
	* sysdeps/unix/sysv/linux/i386/scandir64.c: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc32/scandir64.c: Likewise.
	* sysdeps/unix/sysv/linux/scandir64.c: New file.