[tex-live] Adding installer support for x86_64-cygwin

[Karl Berry]

Hi Ken,

    Does anything else need to be done as far as 
    installation is concerned?

Do you get x86_64-cygwin as the platform identifier in the installer now
(i.e., the defaults for transforming the config.guess output work)?
tlmgr -print-platform or install-tl -print-platform is an easy way to
test, as you may 

Ultimately we will want the xz and xzdec binaries, but that can wait
until after the freeze.

Do you expect to provide asymptote, biber, xindy, as with i386-cygwin?
(Again, nothing to do until after the freeze, but just checking.)

Thanks,
Karl

P.S. For the record, I tersely described the "add a new platform"
process in tlpkg/doc/packages.txt (for some years).