LICENSES/GPL2.0: Add GPL-2.0-only/or-later as valid identifiers

Quite some files have been flagged with the new GPL-2.0-only and
GPL-2.0-or-later identifiers which replace the original GPL-2.0 and
GPL-2.0+ identifiers in the SPDX license identifier specification, but the
identifiers are not mentioned as valid in the GPL-2.0 license file.

Add them to the license file and to the Linux-syscall-note exception to
make everything consistent again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

LICENSES: Add the GPL 2.0 license

Add the full text of the GPL 2.0 license to the LICENSES directory. It was
copied directly from the COPYING file in the kernel source tree as it
differs from the public available version of the license in various places
including the FSF.

Philippe did some research on the GPL2.0 history:

There is NO trustworthy version of an official GPL 2.0 text: the FSF
official texts are all fubar (if only in small and subtle ways). The FSF
texts should be authoritative, but then which one? They published more
GPL 2.0 versions than most. So we would be hard pressed to blame SPDX or
the OSI for having their own minor variant.

Then in digging further, I found the ONE true original GPL with a file
time stamp on June 2 1991, 01:50 (AM?, PM? unknown time zone?) ! in an
old GCC archive.

For the posterity and everyone's enjoyment I have built a git history
of GPL 2.0 Mark1 to Mark6

See https://github.com/pombredanne/gpl-history/commits/master/COPYING

I also added a shorter history of the Linux COPYING text. The first
version in Linus's git tree is based on the very fine and well tuned GPL
2 Mark4, the first fully Y2K compliant version of the GPL 2, as you can
see from the diffs with the former Mark3: that was dangerously stuck in
the last century.

The current version in is based on a rare GPL 2.0 Mark5.1 aka "Franklin
St", that I do not have in my history yet and spells "Franklin St."
rather than "Franklin Street." Therefore there is likely another GPL 2.0
version between Mark4 and Mark5 that I have yet to find and may not have
been caught by the archive.org spiders. Here help and patches welcomed:
this is likely an important missing link.

Further information about this archaelogical research;

http://lkml.kernel.org/r/CAOFm3uEzRMf261+O-Nm+9HDoEn9RbFjH=5J9i1C2GgMUg2G4LA@mail.gmail.com

Add the required tags for reference and tooling.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Reviewed-by: Jonas Oberg <jonas@fsfe.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>