Unix buildinfo: stop saying 'GTK' in pure CLI utilities.

Unix PSCP, PSFTP, Plink and PuTTYgen now just report their build
platform as '64-bit Unix' or '32-bit Unix', without mentioning
irrelevant details of what flavour of GTK the other tools in the suite
might have been built against.

(In particular, they now won't imply anything outright untrue if there
was no GTK present at build time at all!)