Consolidate Linux readdir{64}{_r} implementation

Consolidate Linux readdir{64}{_r} implementation

This patch consolidates Linux readdir{64}{_r} implementation on just
the default sysdeps/unix/sysv/linux/readdir{64}{_r}.c ones.  The
default implementation handle the Linux requirements:

  * readdir{_r} is only built for _DIRENT_MATCHES_DIRENT64 being 0.

  * readdir64{_r} is always built and aliased to readdir{_r} for
    ABI that define _DIRENT_MATCHES_DIRENT64.

  * A compat symbol is added for readdir64{_r} for ABI that used to
    export the old non-LFS version.

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.

	* sysdeps/posix/readdir.c (__READDIR, __GETDENTS, DIRENTY_TYPE,
	__READDIR_ALIAS): Undefine after usage.
	* sysdeps/posix/readdir_r.c (__READDIR_R, __GETDENTS, DIRENT_TYPE,
	__READDIR_R_ALIAS): Likewise.
	* sysdeps/unix/sysv/linux/arm/readdir64.c: Remove file.
	* sysdeps/unix/sysv/linux/arm/readdir64_r.c: Likewise.
	* sysdeps/unix/sysv/linux/m68k/readdir64.c: Likewise.
	* sysdeps/unix/sysv/linux/m68k/readdir64_r.c: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/readdir64.c: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/readdir64_r.c: Likewise.
	* sysdeps/unix/sysv/linux/i386/readdir64.c: Likewise.
	* sysdeps/unix/sysv/linux/i386/readdir64_r.c: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-32/readdir64.c: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-32/readdir64_r.c: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc32/readdir64.c: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc32/readdir64_r.c: Likewise.
	* sysdeps/unix/sysv/linux/wordsize-64/readdir.c: Likewise.
	* sysdeps/unix/sysv/linux/wordsize-64/readdir64.c: Likewise.
	* sysdeps/unix/sysv/linux/wordsize-64/readdir64_r.c: Likewise.
	* sysdeps/unix/sysv/linux/wordsize-64/readdir_r.c: Likewise.
	* sysdeps/unix/sysv/linux/readdir.c: New file.
	* sysdeps/unix/sysv/linux/readdir_r.c: Likewise.
	* sysdeps/unix/sysv/linux/readdir64.c: Add compat symbol if required.
	* sysdeps/unix/sysv/linux/readdir64_r.c: Likewise.