[tlbuild] pdftex rebuild on 8-byte architectures

[Karl Berry]

Unfortunately, it seems we must make new pdftex binaries on 8-byte
platforms. x86_64-linux is the most likely one to matter in practice,
but in principle we should update all amd64-*, x86_64-*, etc. (I don't
know what they all are.)

Hopefully the new binary will create .fmt files which can be shared with
4-byte platforms (i386-*, Windows). If someone has access to a 4-byte
machine, please test. I don't have any easy way to test right now.

I changed the pdftex version to 1.40.23 for this. It also includes two
fixes by Thanh to the (obscure) \letterspacefont stuff, which should not
have be a problem.

As well as trunk being updated, I committed the change to the
texlive2021 branch and made a new texlive-2021.3 tag for
it. Nevertheless, it's fine to compile from trunk, as nothing else
relevant to pdftex has changed (as far as I know).

We should not commit anything but the new pdftex itself. Everything
will have changed, but only pdftex needs to be/should be updated.

Please send me your new pdftex.gz binaries (stripped), or commit
yourself if you wish. Or do your regular update thing and I will extract
pdftex. --thanks, karl.